phys. stat. sol. (c) 3, No. 4, 1217–1220 (2006) / DOI 10.1002/pssc.200564724

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Study on electrical properties of II-VI-based distributed Braggreflectors

Kai Otte∗, Carsten Kruse, Jens Dennemarck, and Detlef HommelUniversity of Bremen, Solid State Physics, Otto-Hahn-Allee 1, 28359 Bremen, Germany

Received 12 September 2005, revised 18 October 2005, accepted 22 October 2005Published online 9 March 2006

PACS 73.61.Ga, 78.66.Ef, 81.05.Dz

The key technology for electrically pumped vertical-cavity surface-emitting lasers (VCSEL) is the realisa-tion of high reflectivity Bragg Mirrors (DBRs) with sufficient conductivity in vertical direction. Several n-and p-doped DBRs, consisting of ZnSSe as high index and a MgS/Zn(Cd)Se superlattice (SL) as low indexmaterial were grown on GaAs substrates. Mesa structures were prepared and furnished with metal contacts.On the one hand a sufficient conductivity of n-doped DBRs has been achieved. The conductivity dependsstrongly on the period and composition of the MgS/Zn(Cd)Se-SL. On the other hand the p-doped DBRsshowed a resistance of several orders of magnitude higher and are unlikely to be used in VCSELs.

1 Introduction Vertical-cavity surface-emitting lasers have several advantages compared to edge-emit-

ting lasers as there are low threshold density, a low divergent beam or single mode emision. These devices

are suited to easy injecction in fibres or single photon emitters. Optically pumped devices operating at

room temperature have been developed by our group [1]. The key technology towards electrically pumped

devices are high reflectivity Bragg-mirrors with sufficient conductivity in vertical direction.

The use of II-VI materials enables an emission in the green spectral region which is of high interrest for

many applications. With green beeing a primary color it can be used together with red and blue emitters for

display technologies. Another application is an optical network using plastic optical fibre with a absorption

minima at 560 nm. A green-emitting VCSEL can therefore enhance the performance of these networks.

In literature approaches of II-VI-DBRs have been developed using ZnSe/ZnS or ZnSe/ZnTe-DBR struc-

tures [2, 3]. Also DBRs wtih ZnSe/MgS-SLs as low index material have been reported but with ZnSe as

high index material and fewer SL-periods [4]. These structures reach reflectivities around 93%.

Another approach for electrical operation is the use of a tunnel-junction in entirely n-doped devices.

This enables the negligence of the more complicated p-doping.

2 Experimental setup All DBRS were grown in a molecular beam epitaxy system. Zinc, magnesium,

and tellur were provided by Knudsen cells, sulfur and selenium by valved cracker cells. The n-type dop-

ing, was achieved using ZnCl2 as dopand. Activated nitrogen as p-dopand was provided by a rf-plasma

source. During growth the sample was investigated by reflection high energy diffraction (RHEED), optical

pyrometer and spectroscopic relectometry.

As substrate GaAs was used. Thermal deoxidation was done prior to the growth start. The samples then

were furnished with metal contacts using 400 nm induim for n-doped DBRs and 20 nm gold and 80 nm

palladium for p-doped DBRs. Mesa structures (500 × 500 µm) were dry-etched by a chemical assisted ion

beam etcher (CAIBE). Then these structures were I–V measured.

∗ Corresponding author: e-mail: otte@ifp.uni-bremen.de, Phone: +49 421 218 2856, Fax: +49 421 218 4581

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1218 K. Otte et al.: Study on electrical properties of II-VI-based distributed Bragg reflectors

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

3 Growth All DBRs were grown lattice matched to the substrate. To achieve this the high index ma-

terial consists of ZnSe with added sulfur. The low index material is a SL of Zn(Cd)Se and MgS. This can

be lattice matched by varying the thicknesses of the layers. All samples were grown at a temperature of

280◦C.

First, n-doped DBRs with different low index layers were grown. Two different low index SLs with

22.5 periods were tested, a ZnSe/MgS-SL consisting only of binary materials and a ZnCdSe/MgS-SL.

Adding cadmium enables a thicker MgS layer while the SL is still lattice matched to the substrate. The

MgS layers have a thickness of 1.5 nm, the ZnSe layers have a thickness of 1 nm while the ZnCdSe

layers can be grown only 0.5 nm thick. This leads to a higher influence of the high bandgap material

MgS. The advantage of the ZnCdSe/MgS-SL is the lower refractive index due to the cadmium. The high

index material is a ZnS0.06Se0.94 layer with a thickness of 48 nm. P-type DBRs were only grown with

ZnSe/MgS-SL as low index material. Both n- and p-doped DBRs were grown with different number of

Bragg-mirros pairs varying from 6 to 17.

4 Reflection measurements A n-DBR with 12 periods shows a reflecivity of 97%, a p-doped sample

with 17 periods reaches 99%. This can be seen in Fig. 1. Figure 2 shows the reflectivity of two DBRs

with different low index SL composition and 15 periods. The one with ZnCdSe/MgS SL has a slighly

higher reflectivity due to the higer index contrast of 0.6 compared to 0.4 (ZnSe/MgS-SL). The theoretical

expected values can be achieved. The DBRs containing cadmium show an increased stopband with due to

the higher index contrast. The optical properties are not affected by the doping.

Fig. 1 Reflectivity measurement of two DBRs with dif-

ferent doping. The n-DBR consists of 12 periods and

reaches a reflectivity of 97% while the p-DBR has a re-

flectivity of 99% at 17 periods.

Fig. 2 Reflectivity measurement of two DBRs with 15

periods and different low index SL composition. The DBR

with the ZnSe/MgS-SL has a reflectivity of 95%, the DBR

with ZnCdSe/MgS as low index material reaches 99%.

phys. stat. sol. (c) 3, No. 4 (2006) 1219

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5 I-V measurements The samples then were I-V measured. Figure 3 shows the measurement of three

DBRs with different low index SL. Two of them with cadmium. These show a drastically decreased

conductivity compared to the third sample with a binary SL. Adding cadmium indeed enhances the optical

properties but also increases the resistance in a way the device is unusable for electrical operation. A n-

DBR with 15 periods and a free carriers concentration of 5 × 1018 cm−3 shows a current density of 0.75

A/cm−2 at 1 V compared to 3.2 × 10−9 A/cm−2 with a p-DBR at the same number of Bragg-mirrors and

3 × 1016 cm−3 as the free carrier concentration . This accords to a reduction of nine orders of magnitude.

Fig. 3 I-V measurement of DBRs with different low in-

dex SL composition. The inner graph shows a magnifica-

tion of a factor of 100 of the current density.

Figures 4 and 5 show I-V measurements of p- and n-doped DBRs with different periods. One can see a

distinct connection between the number of Bragg-pairs and the conductivity.

Fig. 4 I-V measurement of n-doped DBRs with 6 and 12 periods.

Fig. 5 I-V measurement of p-doped DBRs with 8 and 17 periods.

6 Conclusion We have demonstrated high reflectivity n- and p-doped Bragg-mirrors. The doping has

no impact on the optical properties of the mirrors. Using Cd in the low index SL increases the optical

Table 1

Structure Periods Free carrier concentration Current density at 1 V

n-DBR MgS/ZnSe-SL 6 5 × 1018 cm−3 6.1 A/cm−2

n-DBR MgS/ZnSe-SL 12 5 × 1018 cm−3 0.7 A/cm−2

p-DBR MgS/ZnSe-SL 8 3 × 1016 cm−3 6.2 × 10−9 A/cm−2

p-DBR MgS/ZnSe-SL 17 3 × 1016 cm−3 2.8 × 10−10 A/cm−2

properties due to a higher refractive index contrast but reduces the conductivity by nine orders of magni-

tude. The n-doped DBRs with a ZnSe/MgS-SL as low index material shown in this work have sufficient

conductivites for electrical operation. But the p-doped mirrors have a much too low conductivity. Laser

structures will be based on undoped or n-doped DBRs. So a VCSEL with bottom and top n-doped mirror

and tunnel-junction seems to be promising for the green emission wavelength. Electrically pumped II-VI

based VCSEL structures seem to be possible.

References[1] C. Kruse et al., phys. stat. sol. (b) 241, 731 (2004).

[2] T. Tawara et al., J. Cryst. Growth 184/185, 777-782 (1998).

[3] F. C. Peiris et al., Semicond. Sci. Technol. 14, 878-882 (1999).

[4] T. Tawara et al., J. Cryst. Growth 221, 699-703 (2000).

1220 Kai Otte et al.: Study on electrical properties of II-VI-based distributed Bragg reflectors

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com