SPIE Proceedings [SPIE Lasers and Applications in Science and Engineering - San Jose, CA (Saturday 21 January 2006)] Solid State Lasers XV: Technology and Devices - Middle-infrared

Middle-infrared electroluminescence of n-type Cr doped ZnSe crystals

Lawrence Lukea,b, Vladimir V. Fedorova, Igor Moskaleva, Andrew Galliana, Sergey B. Mirov∗a a) Department of Physics, University of Alabama at Birmingham, 310 Campbell Hall, 1300

University Blvd., Birmingham, Alabama 35294, USA b) The University of Massachusetts at Boston

ABSTRACT

We report the study of middle-infrared electroluminescence of n-type, Cr doped bulk ZnSe crystals. n-type, Cr-doped ZnSe samples were prepared in three stages. At the first stage, the undoped polycrystalline ZnSe samples were grown by chemical vapor deposition. During the second stage, the doping of 1 mm thick ZnSe polycrystalline wafers was performed by post-growth thermal diffusion of Cr. Finally, Cr:ZnSe wafers were annealed with Al2Se3 and ZnSe powders in sealed vacuumed ampoules at 950 °C for 96 hours. Comparison of the absorption spectra of the crystals before and after thermal diffusion with Aluminum indicates the preservation of the desired Cr2+ ions. Ohmic contacts for electrical measurements were formed by polishing the facets and wetting the surface of the crystals with In. The best crystals demonstrated conductivity of up to 10-100 ohm*cm. The electroluminescence measurements were taken using synchronous detection methods with an InSb detector. A pulse generator output (100V) at 5 kHz and a lock-in amplifier were used to distinguish luminescence signals from other possible noise sources. We report the observation of middle-infrared (2-3µm and 8µm) and visible (~600 nm) electroluminescence of n-type Cr doped bulk ZnSe crystals. Keywords: Electroluminescence, middle-infrared, Cr:ZnSe, n-type ZnSe, ohmic contacts.

I. Introduction

There has been a long standing demand for compact and tunable mid-IR laser sources operating at or near room temperature. The recent successes in the use of Cr2+-doped zinc chalcogenides under optical pumping as continuous wave or pulsed, broadly tunable mid-IR solid state lasers have been well documented. 1-5 In the hope that these materials might someday be integrated with present day semiconductor technology to create electrically pumpable solid state lasers6 , it is necessary to investigate the occurrence of near to mid infrared luminescence under electrical excitation. We report here, for the first time, an observation of the aforementioned luminescence under electrical excitation from chromium-doped n-type ZnSe.

II. Possible mechanisms of chromium excitation

For decades, the pure and doped (Cu and Mn) wide-bandgap II-VI semiconductors have been called promising materials for fabrication of light-emitting devices and phosphors for electro-luminescent displays. It was noticed more than 50 years ago 7 that inadvertent presence of transition metal (TM) ions such as chromium provides very effective deactivation of visible light emission from donor-acceptor pair (DAP) or intra-shell transitions of Cu and Mn. Cr2+ ions introduce deep energy levels in the forbidden gap, with 5T2 being the ground state and 5E the first excited state (see Figure 1).8 It was determined that in most of the chromium and iron related recombination processes, characteristic intra-center mid-IR emissions of Cr2+ 5E→5T2, with the wavelength ~2µm are induced.9 The nature of these processes of inter-band excitation and subsequent Cr recombination can be quite different. It was found that mid-IR intra-shell photoluminescence (PL) of chromium can be induced due to the following major processes.

∗ [email protected]; phone 1 205 934-8088; fax 1 205 934-8042; http://www.phy.uab.edu/~mirov/

Solid State Lasers XV: Technology and Devices, edited by Hanna J. Hoffman, Ramesh K. Shori, Proc. of SPIE Vol. 6100, 61000Y, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.646935

Proc. of SPIE Vol. 6100 61000Y-1

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a) The first process relates to the binding of excitons by Cr ions. For most cases TM bound excitons decay nonradiatively and energy is transferred to states of impurity, which was binding the exciton. So, the energy transfer results in intra-shell excitation (Cr2+)* and not in ionization of TM ion.10

b) The second process relates to TM excitation caused by energy transfer from adjacent DAP to TM ion leading to TM intra-shell excitation.11 As in case a) the energy transfer results in intra-shell excitation and not in ionization of TM ion.

c) The third process is due to TM2+ionization caused by an Auger-type processes, followed by recapture of the hole or electron by the ionized Cr and as in case a) to mid-IR PL.12

d) The fourth process is a direct excitation of Cr2+ centers in ZnSe by the impact of hot carriers. This mechanism is usually observed in Mn:ZnS TFEL devices.13

e) Another possible mechanism of excitation suggests impact ionization of Cr2+.14 f) The sixth process relates to the fact that carrier trapping by ionized impurity can proceed by one of the excited core

states and thus result in intra-shell emission of TM ion.15,16 The sixth process has been considered in details by Kimpel in 15. Interband optical or electrical excitation generates electron-hole pairs

hνg→e-CB+ e+

VB (1)

Electrons are captured by lattice neutral Cr2+ ions which attain a quasi-negative charge state:

(Cr2+)0 + e-CB+→(Cr+)-+hν1 (2)

The Cr+ with negative effective charge would tend to attract the holes created in reaction (1) with formation of excited state of Cr2+ according to reaction:

(Cr+)- + e-VB+→(Cr2+)*+hν2 (3)

and finally after relaxation of excitation to the 5E excited level radiative transition (mid-IR PL) to the ground state will close the cycle

(Cr2+)* →(Cr2+)0 +hν3 (4)

Alternatively the free holes in the valence bands can be captured by Cr2+ impurities:

CB

VB

ZnSe

5T2 hν

2.7 eV

Cr2+/Cr1+

5E 0.8 eV

0.3 eV

Figure 1. Energy levels of chromium in the forbidden band of ZnSe



(Cr2+)0 + e+

VB+→(Cr3+)++hν4 (5)

Conduction electrons would then recombine with these positively charged Cr3+ levels:

(Cr3+)+ + e-CB+→(Cr2+)*+hν5 (6)

Finally reaction (4) will take place leading to the mid-IR PL. Processes similar to (5) and (6) have been successfully utilized by Klein in 16 where first observation of laser oscillations at 3.53 µm due to intrashell transitions of the Fe2+ centers in n-type InP:Fe under interband optical excitation was reported. Considered mechanisms of chromium excitation are based on recombination of electron hole pairs created during optical interband or electrical in p-n ZnSe structures, or n-type ZnSe in case of avalanche regime of excitation. Electrical excitation of n-type ZnSe in the absence of avalanche could excite Cr2+ centers in ZnSe mainly by the impact of hot carriers with chromium optical centers13 or via impact ionization of Cr2+.14

The information discussed in this paragraph clearly states that Cr belongs to the most active centers of interband recombination leading to intracenter excitation and mid-IR emission.

III. Crystal Preparation

The n-doped Cr:Al:ZnSe crystals were prepared using two stage doping process. Pure ZnSe samples were grown using CVD and were doped by successive post-growth thermal diffusions of Cr and Al, respectively. The sizes of samples were about 1×4×4 mm. During the first thermo-diffusion the ZnSe samples were placed on top of Zn and CrSe powder and annealed in sealed evacuated ampoules (~10-5 Torr) for seven days at 950°C for crystal doping with active Cr2+ ions. Zinc powder was used in the annealing to prevent the formation of Zn vacancies, and CrSe was used to compensate for sublimation of Se.

The n-type samples were obtained by diffusion of Al impurities .17,18 The crystals were placed on top of mixture of Zn and Al powders (97 wt.% of Zn and 3 wt% of Al), and again placed in identical evacuated ampoules. They were annealed for 100 hours at 950°C. At the end of the Aluminum annealing, the samples were quickly cooled by immersion in water.

IV. Optical characterization of Cr:ZnSe and Cr:Al ZnSe The chromium concentration in the samples was controlled using absorption spectrum measured with Shimadzu UV-VIS-NIR 3101PC Spectrophotometer. A typical room temperature mid-infrared absorption spectrum of Cr:ZnSe is depicted in Figure 2 and reveals a peak at 1.75 µm corresponding to the 5T2→5E transition of Cr2+ ions in ZnSe with maximum absorption cross-section σ≈10-18 cm2. After chromium diffusion procedure the average absorption coefficient of Cr:ZnSe samples was about k =4 cm-1, which corresponds to Cr2+ ions concentration N=4×1018 cm3. Subsequent thermo-diffusion of Al in Cr:ZnSe was accompanied by ~ two-fold decrease of the Cr2+ coefficient of absorption. However, Cr2+ absorption could still be observed. One of the reason of the Cr2+ suppression could be compensation of divalent chromium by donor impurities in Cr:Al:ZnSe samples. Another reason relates to necessity to polish crystal faces after thermal diffusion of Al. Since the largest chromium concentration is near the surface of the crystal, its polishing might result in polishing off some amount of chromium.



The emission spectra of 5T2↔5E transition of the Cr:ZnSe and Cr:Al:ZnSe samples under optical excitation are depicted in Figure 2 (B). AS one can see they identical and in a good agreement with the published data.1-6

V. Contacts formation

The procedure of formation of the Ohmic contacts was as follows: in the beginning, the surfaces of the samples were slightly polished and cleaned to avoid surface conductivity effects. After drying, the crystal was heated to about 120°C and liquid Indium was applied to the facet of the crystal. Once the Indium wet the surface and spread evenly over it, the crystals were placed on an uncoated side and heated to about 250°C for several minutes. This heating allowed near-surface diffusion of Indium necessary for proper contact formation.19

If the contact had to be reapplied, the coated sample was placed in ferric chloride and mechanically agitated until the Indium dissolved. Once clean of In, the sample was washed with acetone and the process described above was repeated. The formation of oxide layer was sometimes observed during the application of Indium. It was noticed that once as oxide layer was formed, In would not wet the surface of the crystal. Reapplication of the contact was necessary at this point. The conductivity of the crystals was confirmed by a series of I-V measurements from 0 to 60 DC volts in 10V increments using a Fluke 199 Digital Scopemeter and a Tektronix PS280 DC Power Supply. The same type of I-V characterization was later repeated from 0 to 500 V in increments of 25 V using a Keithley 237 High Voltage Source Measure Unit. The measurement setup involved soldering the crystal into a test circuit where it would be in series with a known resistor. The voltage drop across the known resistor was recorded, and both current and the crystal resistance were calculated later. A reasonable amount of time was allowed (~10s) between each measurement in order to compensate for any capacitance effects of the crystal. The I-V measurements demonstrated that before aluminum doping procedure the Cr:ZnSe sample had high resistivity ρ > 1010 Ωxcm. In n-type Al-doped samples the dark resistivity at room temperature was decreased down to a value of ρ

A

wavelenght, nm

1400 1600 1800 2000

abso

rptio

n, c

m-1

0

2

4

6B

wavelength, nm

1600 2000 2400 2800

Inte

nsity

Figure 2. Absorption (A) and emission spectra of Cr:ZnSe (1) and Cr:Al:ZnSe crystals (2)

1

2

1

2



=103-102 Ω×cm. It is apparent from Figure 3 that the contacts made are not specifically ohmic; there is a Schottky Barrier. However, the I-V curves are sufficiently well-behaved at higher voltages, that we feel justified in proceeding with the experiment.

VI. Electroluminescence The schematic of experimental set-up is depicted in Figure 4. For electrical excitation of luminescence of n-doped Cr:Al:ZnSe crystals a HP 214B Pulse Generator was used. In order to avoid background from blackbody radiation the operation frequency was 1 kHz. Pulses with peak values up to 100V and 19-70µs duration were applied to the samples. Light emitted from the face of the crystal was focused through a CaF2 lens into an Acton Research Corp. SpectraPro-300i Monochromator/Spectrograph. The emission was detected by the detector-EG&G 7260 DSP Lock-In Amplifier- acquisition system combination. We used a photomultiplier in the visible spectral range and near IR. The InSb and HgCdTe detectors were used in the mid IR spectral region.

V-60 -40 -20 0 20 40

I, mA

-8

-6

-4

-2

0

2

4

6

8

Figure 3. Conductivity measurements of two different samples of Cr:Al:ZnSe with In contacts

Acton Research Corp. SpectraPro-300i

F=200Hz 100V

Detector

n- Cr:Al:ZnSe

Lens

HP 214B Pulse Generator

Lock-in Amplifier

Oscilloscope Acquisition System

Figure 4. Schematic diagram of experimental set-up for electroluminescence measurements



Initially signal from electrically excited Cr:Al:ZnSe sample was detected without spectrometer with the use of 2-3 µm bandpass filter and InSb detector. Figure 5 shows typical oscilloscope traces of the voltage across the sample (I) and detected mid IR optical signal (II). Optical signal was measured by InSb detector with the time constant of the detector—preamplifier of ∼0.5 µs. It is clear that the infrared signal observed in the 2-3µm spectral range is due to electrical excitation, as it is perfectly in phase with the voltage across the sample. The encouraging results of Figure 5 were supported by direct measurements of mid-IR electroluminescence with the use of spectrometer. These results are demonstrated in Figure 6(B), where one can clearly see a good coincidence of Cr:Al:ZnSe electrolumiscence (curve (I) with photoluminescence of the same sample measured under direct optical 5T2↔5E excitation (curve – II). Hence, the distinct band in the 1800 – 2400 nm range corresponds to a well known Cr2+ luminescence.1,2 It was also revealed that under electrical excitation in addition to Cr2+ mid-IR electroluminescence there exist

Time, µs-200 0 200

V

-4

-2

(I)

(II)

Figure 5. The oscilloscope traces of the voltage across the sample (I) and mid IR optical signal (II)

A

Wavelength, nm

400 600 800 1000

Inte

nsity

, a.u

.

(I)

B

wavelength, nm

1600 1800 2000 2200 2400 2600 2800

Inte

nsity

, a.u

.

(II)(I)

Figure 6. Visible (A) and Mid-IR (B) emission spectra of n-type Cr:Al:ZnSe under electrical (curve -I) and

direct optical 5T2↔5E excitation (curve – II)



luminescence signals in two other spectral bands. Figure 6A demonstrates a strong visible emission band detected near 600nm. It was visible even with the naked eye. Some authors assign this emission to Vzn-Al complex in conductive crystals.17 In addition to these bands the luminescence band around 8 µm was also observed under electrical excitation. The nature of the 8µm electroluminescence is not yet understood at this time.

VII. Conclusions Room temperature electroluminescence was achieved in n-type Cr:Al:ZnSe. Visible and mid-IR emissions were observed under electrical excitation. The mid-IR electroluminescence over 1800-2800 nm spectral range is in a good agreement with Cr2+ fluorescence under optical excitation. The visible emission observed is in the 600 nm range and is attributed to Vzn-Al complex in conductive crystals. The nature of 8 µm luminescence requires additional studies. Utilization of p-n junction structures should increase efficiency of the energy transfer from the host to Cr2+ ions.

Acknowledgements

We acknowledge support from the Nation Science Foundation (NSF)-Research Experience (REU)-site award to the University of Alabama at Birmingham (UAB) under Grant No. DMR-0243640. This work was also partially supported by NSF grants ECS-0424310 and EPS-0447675.

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SPIE Proceedings [SPIE Lasers and Applications in Science and Engineering - San Jose, CA (Saturday 21 January 2006)] Solid State Lasers XV: Technology and Devices - Middle-infrared