III-Nitride avalanche photodiodes Patrick Kung, Ryan McClintock, Jose Luis Pau Vizcaino, Kathryn Minder, Can Bayram, and

Manijeh Razeghi*

Center for Quantum Devices; Department of Electrical Engineering and Computer Science; Northwestern University; Evanston, Illinois 60208; USA

ABSTRACT Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first back-illuminated GaN p-i-n APD structures on transparent sapphire substrates. The 25 µm × 25 µm device characteristics were measured, and compared with the same devices grown on GaN templates, under low bias and linear mode avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be greater than those of electrons. These APDs were also successfully operated under Geiger mode. Keywords: GaN, Avalanche, Photodiode, APD, Back-illuminated, Impact ionization coefficients

1. INTRODUCTION Although known for most of the 20th century, wide bandgap III-Nitride semiconductors have only been the subject of intense scientific and technological developments since the 1990’s, which was primarily driven by the quest for blue lasers and high brightness visible light emitting diodes. Their potential for devices operating in the ultraviolet (UV) had then remained largely un-explored, until a few years ago when there was renewed interest in AlGaN compounds for UV light emitting devices, and especially for UV photodetectors that are visible-blind or solar-blind in order to take advantage of the lower natural background on Earth in the UV spectral region. This detector feature would dramatically relax system requirements for expensive and efficiency limiting optical filters to remove undesired out-of-band photons. From an applications standpoint, such UV photodetectors would be useful in chemical and biological agent spectroscopy and threat detection, non-line-of-sight communications, flame detection and monitoring, environmental monitoring, astronomy, early missile threat warning, and covert space-to-space communications.1,2,3 Alongside their intrinsic visible- or solar-blind advantages, AlGaN based photodetectors are also ideally suited for UV photon counting applications as a result of their predicted much higher detective quantum efficiency -a measure of the ability of a photodetector to convert photons into electrons and a key characteristic for photon counting- over any other existing technology.3 Being able to achieve photon counting is motivated by the need for a semiconductor-based alternative to detectors and imaging arrays based on photomultiplier tubes (PMTs), which exhibit very high sensitivity by taking advantage of internal gain (typically ~106), but are at the same time bulky, fragile, contain glass and require large biases (typically 1000 V) to operate effectively.4 Thanks to continued improvements in the quality of wide bandgap AlGaN semiconductors, it has now been possible to demonstrate efficient UV photodetector devices5,6,7, focal plane arrays8 and recently avalanche photodiodes9,10,11,12,13 based on these materials. To date, all reported GaN APD devices were designed for front-illumination operation with photons reaching the p-layer first. This configuration has been mostly used historically because one would get fewer defects by growing a device structure on (several micron) thick n-type GaN templates on sapphire or even GaN substrates. Furthermore, front-illuminated devices are easier to fabricate into individual detectors. By contrast, a back-illumination scheme is essential in the realization of hybridizable APD arrays. In the case of III-Nitrides, this is much more difficult to realize than front-illuminated devices because of the need for high * razeghi@ece.northwestern.edu; phone 1 (847) 491-7251; fax 1 (847) 476-1817; http://cqd.ece.northwestern.edu

Quantum Sensing and Nanophotonic Devices IV, edited by Manijeh Razeghi, Gail J. Brown, Proc. of SPIE Vol. 6479, 64791J, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.713774

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quality of AlN templates/substrates and highly doped AlGaN templates with large Al content in order to maintain optical transparency for the active structure. The spectral bandwidth would also be reduced as a result of absorption by the template layers. The few reports of back-illuminated III-Nitride APDs have been exclusively for solar-blind AlGaN detectors. There is nevertheless a strong scientific and technological desire to investigate back-illuminated GaN avalanche photodiodes for a number of reasons in addition to the need to realize GaN APD arrays. First, it is believed that a thick GaN template or substrate, although preferred, is not a necessity as long as high quality GaN materials can be achieved on AlN templates. And secondly, A back-illuminated p-i-n GaN structure, in which photons reach the n-layer first, would allow to take advantage of hole initiated multiplication, since hole impact ionization coefficients are potentially higher than the electron coefficient in GaN.14

2. MATERIAL GROWTH AND DEVICE PROCESSING

2.1. Material Growth The basics of the material growth and device structure are similar to those discussed earlier.8,15,16 The material was grown in an AIXTRON 200/4-HT horizontal flow low-pressure metalorganic chemical vapor deposition reactor. Double side polished basal plane (00.1) sapphire was used as the substrate in order to allow for the realization of back-illuminated photodetectors. The key to achieving high quality back-illuminated GaN APDs consists of first being able to attain the highest quality AlN epilayers possible and then GaN epilayers. To do so, growth on the sapphire substrate was nucleated with a thin 200 Å low-temperature AlN buffer layer. On top of this a 0.5~0.6 µm thick high quality AlN template layer was grown by atomic layer epitaxy17 at a temperature of ~1300°C. Figure 1 (left) shows a representative atomic force microscopy (AFM) imaging of the resulting AlN layer. The rms surface roughness was less than 1 Å for a 5×5 µm2 scan area. Figure 1 (right) shows the room temperature photoluminescence of such AlN epilayers, confirming the high quality of the films. Open detector x-ray diffraction measurements yielded a (00.2) symmetric peak linewidth of ~40 arcsec and a (10.5) asymmetric peak linewidth of ~240 arcsec. At the origin of this high quality AlN is the good control of the buffer/substrate interface in spite of an existing 13.2 % effective lattice mismatch. Theoretical crystallographic models of such interface have been developed earlier18,19 to determine how the lattice mismatch would be relaxed through the periodic formation of misfit dislocations every 9 atomic planes of sapphire and 8 atomic planes of AlN along the <10.0> direction of AlN. The modeled cross-section is shown in Figure 2 (left). This model was experimentally confirmed through imaging of the cross section between the AlN buffer and the basal plane sapphire substrate using high resolution transmission electron microscopy (HRTEM), Figure 2 (right).

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Figure 1. (Left) Atomic force microscopy imaging of the surface of a high quality AlN template layer grown on basal plane sapphire substrate. The vertical scale is 1 nm. (Right) Room temperature photoluminescence of a high quality AlN layer (inset shows magnified scale).

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Figure 2. (Left) Schematic crystallographic model of the cross-section between AlN and basal plane sapphire along the <10.0> direction of AlN. (Right) High resolution cross-section TEM imaging of the same interface.

The quality of GaN epilayers grown on such AlN templates was subsequently assessed. GaN films with 0.5~1.5 µm thickness were deposited under conditions similar to those reported earlier.15 The layer surfaces were very smooth with very few observable dislocation termination pits, as shown in AFM imaging in Figure 3. The rms surface roughness was only 1.29 Å for a 5×5 µm2 scan area. Open detector x-ray diffraction measurements yielded a (00.2) symmetric peak linewidth of ~82 arcsec as shown in Figure 4 (left). The typical room temperature photoluminescence spectrum was measured using a frequency doubled argon-ion laser (λ=244 nm) and is shown in Figure 4 (right). The peak luminescence occurred at 361 nm with a linewidth of ~40 meV and no yellow defect related luminescence was observed. Undoped GaN epilayers exhibited a room temperature mobility of 450~500 cm2/V·s for a residual carrier concentration of n~1016 cm-3. N-type and p-type doping of these layers yielded carrier concentrations of 1~2×1018 cm-

3.

Figure 3. Atomic force microscopy imaging of the surface of a high quality GaN grown on an AlN template. The vertical scale is 2.2 nm.

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Figure 4. (Left) Open detector x-ray diffraction spectrum and (Right) room temperature photoluminescence spectrum of a 1.5 µm GaN epilayer grown on high quality AlN template on sapphire substrates.

These calibrated GaN epilayers were used as building blocks for the back-illuminated GaN p-i-n avalanche photodiodes reported here. The structures consisted of a p-type GaN epilayer, on undoped i-GaN on n-type GaN, all grown on the high quality 0.5~0.6 µm AlN template on sapphire. Several devices were realized with different GaN layer thicknesses. The thickness of the p-type GaN:Mg layer ranged from 100 to ~300 nm, while the thickness of the undoped i-GaN region was varied from 50 to 200 nm, and the n-type GaN:Si layer thickness ranged from 100 to 400 nm. These devices will be referred to as type-A APDs. Although designed for back-illumination, they can be measured under both back- and front-illumination. Unlike other reports of avalanche photodiodes, the total thickness of these devices, including template, did not exceed 1.5 µm. Similar GaN p-i-n structures were also realized on 3 µm thick n-type GaN:Si templates on sapphire for comparison purposes. These samples will be referred to as type-B APDs. Unlike their type-A counterparts, type-B photodiodes can only be operated under front-illumination because of the high optical absorption from the underlying GaN:Si template layer.

2.2. Device Processing All samples were first rapid thermally annealed at 1000 ºC for 30 seconds under dry N2 to activate the magnesium in the p-type GaN:Mg layers. The material was patterned into an array of 25 µm × 25 µm square mesas using electron cyclotron resonance (ECR-RF) dry etching to reach the n-type GaN:Si contact layer. A thin 30 Å Ni / 30 Å Au layer was deposited on top of the mesas and annealed under ambient air at 500 ºC for 10 minutes in order to form ohmic contacts to the p-type material on top of the device. A 400 Å Ti / 1200 Å Au metal layer was deposited on the GaN:Si layer to form the common n-type contact and on top of the thin Ni/Au as a thick metal contact. The devices were finally covered with 300 nm of SiO2 deposited by plasma enhance chemical vapor deposition to help protect the mesas and prevent premature breakdown of the devices; windows were opened via wet etching. An illustration of a typical single diode out of the array is shown in Figure 5.

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Figure 5. Scanning electron micrograph of an APD after processing. The common n-contact, not shown, is far removed from the mesas to avoid air breakdown of the devices. Indium bumps are also shown on these devices making them suitable for fabrication of APD arrays.

3. LOW BIAS DEVICE CHARACTERISTICS

3.1. Current-voltage characteristics of type-A and type-B photodiodes A typical current-voltage(I-V) curve of a type-A photodiode measured in the dark under small applied bias is shown in Figure 6. The diodes exhibited a turn on voltage of 4.4~6.2 V. The diodes had very low dark currents at the ~10 fA noise floor limit of the curve tracer used to record these I-V curves, up to ~10 V reverse bias. The ideal diode equation, shown below, was used to fit the forward bias data.

)exp(nkTqVCI f ×= Equation 1

where q is the charge of an electron, k is the Boltzmann constant, T is the temperature, and C a constant. This fitting allows the extraction an ideality factor, n, which can provide insight into the conduction mechanisms operating in these photodetectors. The ideality diode equation arises from the combination of the equations for diffusion current and recombination current: the two currents that usually dominate the diode current, and the ideality factor is expected to have a value between 1 and 2. Whereas if n is closer to 1, then diffusion current dominates and if n is closer to 2 then recombination current dominates. The ideality factor in type-A photodiodes ranged from a little below 2 to 4. The series resistance of these type-A diodes was calculated and yielded values ranging from 450 to 3000 Ω, and scaled with the inverse of the n-type GaN layer thickness, which suggests that there was not a significant parallel conduction path contribution from the GaN/AlN interface. A typical I-V characteristic of a type-B photodiodes measured in the dark under small applied bias is shown in Figure 7. The diode turn on voltage was about 4.2 V. These devices also exhibited a very low dark current up to ~10 V reverse bias. By contrast to type-A diodes, the ideality factor of the diodes on GaN templates were smaller, ranging from 1.6 to 2.7. Furthermore, type-B diodes had a much lower series resistance, in the range of 29~190 Ω, which was due to the much larger overall GaN:Si layer thickness.

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Figure 6. Current-voltage characteristics of a type-A photodiode (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN) in linear and logarithmic scale (inset). The turn-on voltage was ~6.2 V, ideality factor was ~3.32, and series resistance was ~521 Ω.

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Figure 7. Current-voltage characteristics of a type-B photodiode (same device structure as in Figure 6 but on GaN template) in linear and logarithmic scale (inset). The turn-on voltage was ~4.28 V, ideality factor was ~2.45, and series resistance was ~39.4 Ω.

3.2. Spectral response The spectral response was measured for type-A photodiodes under back-illumination. The experimental apparatus involved a xenon arc lamp attached to a monochromator. To allow determination of the absolute responsivity, the system was first calibrated with a UV-enhanced silicon photodetector masked with a precision calibrated aperture. After calibration of the system, the photoresponse was recorded with a transimpedance amplifier, the output of which was fed to a lock-in amplifier for synchronous detection.

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Typical photoresponse and external quantum efficiency spectra are shown in Figure 8 for a (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN) structure. The photodiodes were visible-blind with an unbiased peak responsivity of 46.5 mA/W at 363 nm, which corresponds to a value of 15.9 % for the external quantum efficiency. Interestingly, there was a pronounced excitonic absorption peak at 363nm, which is under further investigation.

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Figure 8. (Left) Unbiased spectral responsivity shown in log scale and (Right) external quantum efficiency shown in linear scale of a type-A photodiode (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN).

4. LINEAR MODE AVALANCHE OPERATION

4.1. Current-voltage and gain characteristics for type-A APDs under high reverse bias The I-V curves of type-A photodiodes were then recorded under large reverse biases. In the case of a (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN) structure for example, after about 10 V the dark current came out of the noise floor and began to increase. The same measurements under illumination with broadband white light from the xenon lamp showed a large photocurrent that increased moderately with bias. Avalanche breakdown occurred at a reverse bias of 78 V. No microplasma was observed at that point. Both the dark and illuminated I-V curves are plotted in log scale below in Figure 9. These measurements are generally non-destructive, and the same device is consistent from measurement to measurement. The figure below includes error bars from multiple alternating measurements in the illuminated and dark conditions. Using a 1D finite element model20, with the layer thicknesses and doping concentrations from the epitaxial growth as input parameters, we were able to simulate the electric field build-up in the multiplication region and determine that the breakdown electric field corresponding to the breakdown voltage of 78 V was ~2.7 MV/cm. The avalanche gain (M), defined as the difference between the primary multiplied current and the multiplied dark current, normalized by the difference between the primary unmultiplied current and the unmultiplied dark current, was calculated from the previous I-V characteristics in Figure 9 and using Equation 2 below.

[ ][ ] edunmultipli

multiplied

darkdIlluminate

darkdIlluminate

I-II-I

M = Equation 2

The 1D finite element model was used to help determine the primary unmultiplied photocurrent and the unmultiplied dark current that define where gain M equals 1. The unmultiplied currents were then taken from that point and the corresponding gain curve was calculated and shown in Figure 10. A very sharp increase in gain occurs at the avalanche breakdown and gain values reaching nearly 1500 were obtained.

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Figure 10. Calculated avalanche gain curve from the data of Figure 9 for a type-A APD (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN) in linear scale and logarithmic scale in the inset.

4.2. Current-voltage and gain characteristics for type-B APDs under high reverse bias Similarly, the dark and illuminated I-V curves from type-B photodiodes were recorded and are plotted in log scale below in Figure 11 for the same active region as discussed previously, i.e. (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN). The illuminated measurements were carried out under front-illumination conditions this time. The dark current came out of the noise floor and began to increase after a reverse bias about 18 V. The photocurrent also increased moderately with bias before reaching avalanche breakdown at a reverse bias of 79 V. No microplasma was observed at that point either. The corresponding breakdown electric field was modeled to be the same ~2.7 MV/cm as for the type-A structure. This suggests that the breakdown electric field depends only on the thickness of the i-region

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and not which template is used. The gain curve was calculated in the same manner as previously and is shown in Figure 12 with gain values in excess of 1500.

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Reverse Bias (V) Figure 11. Current-voltage behavior as a function of applied reverse bias, both under illumination and in the dark, for a type-B APD (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN). Errors bars have been added to indicate the variation over several consecutive pairs of alternating light-dark measurements of the same diode.

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Figure 12. Calculated avalanche gain curve from the data of Figure 10 for a type-B APD (~300 nm p-GaN/150 nm i-GaN/400nm n-GaN) in linear scale and logarithmic scale in the inset.

4.3. Breakdown electric field in GaN APDs A series of devices were grown with different i-GaN region thicknesses, allowing us to plot the breakdown voltage versus i-GaN thickness in Figure 13. By linearly fitting the data, a breakdown electric field of 2.73 MV/cm was determined. This experimental value is consistent with values obtained from the finite element models carried out, as illustrated earlier in this paper, and with reported values in the literature21,22.

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4.4. Comparison of hole and electron initiated multiplication in type-A ADPs Under back-illumination operation, injected holes would be more likely to initiate the multiplication process and an associated hole multiplication factor could be determined as a function of reverse bias. By contrast, under front-illumination conditions, injected electrons would be more likely to initiate the multiplication and an electron multiplication factor could be determined. To do so under our experimental constraints, i.e. in order to be able to inject sufficient electrons under a front-illuminated configuration, a high intensity UV laser was used instead of a xenon lamp to overcome the optical loss from the thick metal contact on top of the mesas and the shadowing from measurement electrical probes. Comparing the multiplication factors for holes and electrons, we found that the multiplication process occurred at a lower voltage for hole initiated avalanche (i.e. back-illumination) than for electron initiated processes (i.e. front-illuminated) in the same device. This observation, along with noise measurement analysis which will be reported elsewhere23, confirms that the hole impact ionization coefficient is larger than the electron impact ionization coefficient in GaN. This implies that multiplication is dominated by hole initiated impact ionization and that back illumination may yield lower excess multiplication noise.

5. GEIGER MODE AVALANCHE OPERATION These type-A avalanche photodiodes were successfully operated under Geiger mode. The diodes were placed in a simple passive quenching circuit shown in Figure 14 (Top). The APD was DC biased (VDC) slightly below avalanche breakdown and pulse biased above breakdown with VP (i.e. VDC+VP above breakdown) at a 10 kHz repetition rate for a 5 µs pulse duration through a 50 nF coupling capacitor. The photon source consisted of a xenon lamp coupled with a band pass filter at 340 nm. Figure 14 (Bottom) shows a typical pulse current generated when the device undergoes avalanche breakdown. The magnitude of the peak current increases with the DC and the pulse biases. By using neutral density filters, the incident optical power could be reduced. The devices were successfully operated under Geiger mode for very low photon fluxes down to 110 photons/pulse, demonstrating photon counting capability for these APDs.

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Figure 14. (Top) Schematic diagram of the passive quenching circuit used for Geiger mode operation of type-A APDs. (Bottom) Typical current pulse generated when the APD undergoes avalanche breakdown and quenching. The pulse width was 5 µs, for an incident optical power of 6.4 nW at 340 nm.

6. CONCLUSION We have demonstrated the epitaxial growth of device quality GaN layers on high quality AlN templates suitable for the first back-illuminated p-i-n APD structures on transparent sapphire substrates. Square 25 µm × 25 µm APDs were fabricated, their characteristics measured under both low bias and linear mode avalanche operation; they were also compared with the same structures grown on GaN templates. A gain in excess of 1500 was observed after avalanche breakdown and a breakdown electric field of 2.73 MV/cm was determined for GaN, a value consistent with the literature. The hole impact ionization coefficients were experimentally determined to be higher than those for electrons, suggesting multiplication is dominated by hole initiated impact ionization, and that back-illumination may yield lower excess multiplication noise. These APDs were also successfully operated under Geiger mode when incorporated in a simple passive quenching circuit. Geiger mode operation was even observed for very low photon fluxes down to 110 photons/pulse, demonstrating photon counting capabilities of these APDs.

ACKNOWLEDGEMENTS J.L. Pau Vizcaino acknowledges the support of the Fulbright Association and the Spanish Ministry of Education and Science. The authors would like to acknowledge Dr. Donald Silversmith (AFOSR), Dr. Henryk Temkin (DARPA), and Dr. Michael Gerhold (ARO) for their support and encouragement.

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