Photoluminescence characteristics of nitrogen atomic-layer-doped GaAs grown by MOVPE

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CRYSTAL GROWTH

Journal of Crystal Growth 170 (1997) 372-376

Photoluminescence characteristics of nitrogen atomic-layer-doped GaAs grown by MOVPE Hisao Saito *, Toshiki Makimoto, Naoki Kobayashi NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya. Atsugi-shi, Kanagawa 243-01, Japan

Abstract

Nitrogen atomic-layer-doped and uniformly doped GaAs were grown by MOVPE using dimethylhydrazine on a (001) plane. They showed several sharp photoluminescence lines with a full width at half maximum less than 1 meV at 8 K. Compared with uniformly doped GaAs, the photoluminescence intensity of the nitrogen-related line at the longest wavelength is enhanced in nitrogen atomic-layer-doped GaAs, suggesting that it is easier to form nitrogen pairs during atomic layer doping. To investigate the sharp nitrogen-related lines, we also grew GaAs with double atomic-layer-doped planes and varied the distance between the two planes. When the two planes are brought close to 1 nm, two new lines, NN c and NN D, appear between the two nitrogen-related lines, N N A and NN B, observed in a single nitrogen atomic-layer-doped GaAs. The NN c and NN D lines are also observed in uniformly doped GaAs. Therefore, NN a and NN B originate from excitons bound to pairs of nitrogen atoms, both of which are in the (001) plane, while NN c and NN D originate from those bound to pairs of nitrogen atoms, of which pairing directions are not included in the (001) plane. From the photoluminescence characteristics, distances between nitrogen atoms of a pair are estimated for each line. 1. I n t r o d u c t i o n

It is well known that nitrogen (N) atoms act as isoelectronic traps in GaP [1]. An isolated N atom binds an exciton, while two N atoms close together bind an exciton more tightly. As the distance between the N atoms of a pair increases, the binding energy decreases. Therefore, N atoms in GaP show many sharp photoluminescence (PL) lines at low temperatures. The deepest line corresponds to closest-neighbor N pairs, the second deepest to secondneighbor pairs, and so on. The shallowest line corresponds to an isolated N atom. While a similar pair system was reported for the defect induced excitons in undoped GaAs grown by M B E [2,3], there are

* Corresponding author. Fax: + 81 462 40 4729.

only a few studies for a N pair system in N-doped GaAs. For uniformly N-doped GaAs, as the N atom concentration increased, sharp N-related PL lines were observed [4,5]. Recently, we reported on N atomic layer doping into GaAs [6,7] and into AIG a A s / G a A s single quantum wells [7,8] using atomic N in MBE. These samples showed strong and sharp PL lines from excitons bound to N atoms at low temperatures. However, the origin of N-related lines, especially the relationship between the PL wavelength and the distance between the N atoms of a pair, is not clear yet. In this work, we first grew uniformly N-doped G a A s using dimethylhydrazine (DMHy) by M O V P E to find the optimal growth condition for atomic layer doping. Then, we grew N - A L D GaAs and the double-ALD-plane structures. On the basis of the results, we will discuss the origin of the N-related lines in GaAs.

0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)005 23-4

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H. Saito et al./ Journal of Co,stal Growth 170 (1997) 372-376

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2. Experimental procedure

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GaAs layers were grown on (001) semi-insulating Cr-O-doped GaAs substrates by MOVPE using triethylgallium (TEG) and arsine (AsH 3) in hydrogen carrier gas. The TEG partial pressure was fixed at 1.2 X 10 3 Torr, corresponding to the growth rate of 0.40 txm/h above 550°C. It decreased to 0.32 i x m / h at the substrate temperature of 450°C due to incomplete decomposition of TEG. DMHy was used as a N doping gas, because it might decompose into active N radicals more easily than NH 3. The DMHy partial pressure and the reactor pressure were 1.2 × 10 -2 and 76 Torr, respectively. The growth temperature was changed from 450 to 700°C, while the AsH 3 partial pressure was changed from 1.1 X 10 -2 to 5.3 X 10 -2 Yorr to find an optimal condition for atomic layer doping. For N-ALD GaAs, undoped GaAs was grown using TEG and AsH 3 first. Next, after stopping TEG and AsH 3 flows, DMHy was supplied for several seconds onto the GaAs surface to perform atomic layer doping. After doping, an undoped cap layer was grown on the surface so as to form the N-ALD plane in the epitaxial layer. For uniformly N-doped GaAs, TEG, AsH 3, and DMHy were simultaneously supplied onto GaAs substrates. PL measurements were performed at 8 K using an Ar laser operating at 488 nm with an excitation power density of 0.1 roW. In these measurements, GaAs and InGaAs photomultipliers and a double grating monochromator were used. The spectral resolution was about 1 meV. The N atom concentration was determined using secondary ion mass spectrometry (SIMS) analysis.

3. Results and discussions 3.1. N doping characteristics by DMHy First, to find an optimal condition for atomic layer doping, the N doping characteristics were studied in uniformly N-doped GaAs. Fig. 1 shows the N doping concentration as a function of AsH 3 partial pressure in uniformly N-doped GaAs. The growth temperature and the DMHy partial pressure were 600°C and 1.2 × 10 2 Torr. As the AsH 3 partial pressure increases, the incorporated N atom concentration de-

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creases. One possible reason for this is that it is difficult to replace As atoms with N atoms on the GaAs surface when the AsH 3 partial pressure is high. A similar result is observed in N doping into GaP [9]. From Fig. 1, it was found that it is necessary to supply DMHy on GaAs surfaces without AsH 3 to obtain an efficient N sheet concentration in atomic layer doping. Fig. 2 shows the growth temperature dependence of N doping concentration in uniformly N-doped GaAs. The AsH 3 and DMHy partial pressures were fixed at 1.1 × 10 -2 and 1.2 × 10 -2 Torr, respectively. As the temperature decreases from 700°C to 450°C, the N concentration increases, indicating that use of lower growth temperatures is efficient for N doping into GaAs in this temperature range. This trend is also similar to N doping into GaP [9]. At lower temperatures, the AsH 3 cracking efficiency decreases so N atoms are efficiently incorporated into GaAs. Therefore, the N concentration increases with a decrease in temperature, even though DMHy cracking efficiency might slightly decrease at lower temperatures. Above 600°C, the N doping concentration decreases rapidly. One possible reason for this is the N desorption from the surface, as reported for N doping in MBE [10]. Below 500°C, crosshatched patterns were observed due to the lattice mismatch at high N doping concentrations. To find a compromise between GaAs quality and N doping efficiency, we

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H. Saito et al. / Journal of Crystal Growth 170 (1997) 372 376 1023

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( l / T ) of 0.20 S - 1 . This desorption rate is much higher than the reported value at 645°C in MBE (6.7 × 10 4 s L) [10,11]. We found that H radicals remove N atoms on GaAs surfaces [11]. Therefore, the observed saturation in Fig. 3 might be due to the etching effect of N by H radicals created during DMHy decomposition.

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used the relatively low growth temperature of 550°C for N-ALD GaAs growth. Next, we studied the N doping characteristics in N-ALD GaAs. The single N-ALD plane was inserted at the center of the 0.6 txm thick undoped GaAs. The growth temperature was 550°C and DMHy was supplied after stopping TEG and AsH 3 flows to obtain higher N sheet doping concentration. The DMHy partial pressure was fixed at 1.2 × 10 2 Tort. The N sheet doping concentration was determined from the integration of a N peak in a SIMS profile. Fig. 3 shows the N sheet concentration as a function of doping time. The N concentration increases with time and then saturates. The solid line in Fig. 3 shows the calculated results with the desorption rate

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Fig. 4 shows PL spectra for N-ALD and uniformly N-doped GaAs at 8 K. The N-ALD GaAs was grown at 550°C and DMHy was supplied for 2 s without AsH 3 flow for atomic layer doping. The N sheet concentration in N-ALD GaAs was 4.6 × 10 ~2 cm 2, corresponding to the average N atom distance of 4.7 nm. The uniformly N-doped GaAs was grown under the AsH 3 partial pressure of 1 × 10 2 Torr in Fig. 1. Its N volume concentration was 1.5 × 10 19 cm 3, corresponding to the average N atom distance of 4.1 nm. Therefore, both samples in Fig. 4 have a similar N atom distance. In both spectra, GaAs freeand bound-exciton lines disappear and several strong and sharp N-related lines with a full width at half maximum (FWHM) less than 1 meV are observed. For uniformly N-doped GaAs, three strong and sharp lines, labelled as NN c, NN E, and NNI> are observed at 848.0, 850.3, and 855.6 nm, respectively. These lines were observed above 1X 1018 c m 3, while their PL intensities decreased rapidly with the decrease in N doping concentration. At lower N concentrations, we observed several additional lines be-

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H. Saito et a l . / Journal of Crystal Growth 170 (1997) 372 376

tween 822 and 840 nm. The 822 nm line is the shallowest one among the N-related lines. It was also observed in GaAs grown by MBE at lower N concentration and was thought to originate from an exciton bound to an isolated N atom [6]. We will report the PL characteristics of these shallow lines in a separate paper and describe those of the relatively deep lines above 840 nm in this paper. Above the N concentration of 1 X 10 20 cm -3, no PL lines were observed between 800 and 900 nm. This might be due to the band formation of GaAsN alloys, as reported in GaPN [12]. In contrast, for N-ALD GaAs, two strong and sharp N-related lines are observed at 840.1 and 868.1 nm, which are labelled as NN A and NN B, respectively. By the PL measurements using an lnGaAs photomultiplier, it was found that the NN B line was the deepest one. The NN B line is, therefore, thought to originate from excitons bound to the closest-neighbor N pairs. Even though the number of NN B might be much smaller than that of NNA, the PL intensity of NN B is relatively strong because of the tunneling effect of excitons from shallower states to deeper states such as NN B [13]. Compared with uniformly N-doped GaAs, the PL intensity of NN B is enhanced in N-ALD GaAs, suggesting that it is easier to form N pairs during atomic layer doping. In the uniformly N-doped GaAs, NN c, NN e, and NN D are dominant instead of NN A, suggesting that NN c, NN E, and NN D are inherent in uniform doping, that is, they come from three-dimensional pairing of N atoms. Therefore, the NN A line is due to an exciton bound to a pair of N atoms,

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both of which are included in the (001) plane. In contrast, the NN c, NN E, and NN D lines are due to those bound to pairs of N atoms, of which pairing directions are not included in the (001) plane. The possible pairing directions for NN A are [110], [200], [220], [400], [330], [420] and so on, while those of NNc, NN E, and NN D are [211], [222], [321] and so on. The directions denoted titan] are expressed in the coordinate of one member of a pair with the other at the origin, and they denote the integral multiples of half of the unit cell in the cubic GaAs lattice (0.283 nm). To investigate the origin of these N-related lines further, we grew GaAs with double ALD planes. The inset in Fig. 5 illustrates this structure. Two N-ALD planes were inserted while changing the distance (d) between the two planes. Fig. 5 shows the PL spectra for three different distances; d = 1, 3, and 30 nm. The PL spectra for d = 3 and 30 nm are similar to that for the single N-ALD structure, as shown in Fig. 4, indicating that each spectrum is composed of the PL emission from two independent ALD planes. In the PL spectrum for d = 1 nm, however, NN A almost disappeared while NN B was unchanged. In addition, NN c and NN D, which were observed in uniformly N-doped GaAs, appeared. These results suggest that NN c and NN D come from excitons bound to a pair of N atoms in different ALD planes and that they weaken the intensity of N N a due to the tunneling effect of excitons from NN a to NN c and

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H. Saito et al. / Journal of Crystal Growth 170 (1997) 372-376

Table 1 Spectral position of the NNg lines at 8 K and their properties Wavelength (nm) E (eV) Label Direction Separation (nm) 840.1 848.0 850.3 855.6 868.1

1.4760 1.4623 1.4583 1.4493 1.4284

NN A NN c NN E NN o NN B

[420] [321] [222] [211] [ll0]

1.264 1.058 0.979 0.692 0.400

NN D. This result supports the assumption that the NN c and NN D lines are due to excitons bound to N atoms, of which pairing directions are not included in the (001) plane. They also suggest that the distance between the N atoms of a pair for NN c and NN D is around 1 nm. Considering that the binding energy decreases with the increase in distance between the N atoms of a pair, the most probable pairing directions for NN c, NN E, and NN D are [321], [222], and [211], respectively. The binding energy for NN A is the smallest of the N-related lines in Figs. 4 and 5, so the possible pairing directions for NN A are [400], [330], or [420], and so on. Considering the number of equivalent sites and the relatively large energy difference between NN A and NN c (13.7 meV) compared with the difference between NN c and NN E (4.0 meV), [420] is the most probable. Fig. 6 illustrates pairs of N atoms in the GaAs with double ALD planes, and Table 1 summarizes the spectral position of the NNg lines, the most probable pairing direction of N atoms, and the distance between N atoms of a pair.

4. Conclusions N-ALD and uniformly N-doped GaAs were grown by MOVPE using DMHy. The N doping efficiency increases with the decrease in growth temperature and AsH 3 partial pressure. Both N-ALD and uniformly N-doped GaAs layers showed several strong and sharp PL lines with FWHM less than 1 meV. Compared with uniformly N-doped GaAs, the PL intensity of NN B, the N-related line at the longest wavelength, is enhanced in N-ALD GaAs. This result suggests that it is easier to form N pairs during atomic layer doping. We also grew GaAs with double N-ALD planes and varied the distance between

the two planes. Their PL characteristics suggest that NN c and NN D come from excitons bound to a pair of N atoms in different ALD planes and that the NN B line most likely originates from excitons bound to the closest-neighbor N pairs. They also suggest that the distance between N atoms of a pair for NN c and NN D is around 1 rim. The most probable pairing directions of N atoms were [420], [321], [222], and [211] for NNA, NN c, NN E, and NN D, respectively. The corresponding distances between N atoms of a pair were also estimated to be 1.264, 1.058, 0.979, and 0.692 nm.

Acknowledgements The authors would like to thank Dr. Yoshiji Horikoshi and Dr. Yoshikazu Homma for their valuable discussions and comments. We also thank Dr. Tetsuhiko Ikegami for his encouragement throughout this work.

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