InP(0 0 1) MQWs grown by tert-butylarsine in a MOVPE apparatus

Journal of Crystal Growth 248 (2003) 149–152

High-quality, Cr-doped InGaAs/InP(0 0 1) MQWs grown by tert-butylarsine in a MOVPE apparatus G. Cartaa,*, A. D’Andreab, F. Ferna! ndez-Alonsoc, A. Francoc, N. El Habraa, M. Righinic, G. Rossettoa, D. Schiumarinib, S. Selcic, P. Zanellaa b

a CNR-Istituto di Chimica Inorganica e delle Superfici, Corso Stati Uniti 4, I-35127 Padova, Italy CNR-Istituto di Metodologie Inorganiche e dei Plasmi, c.p.10, I-00016 Monterotondo Scalo (Rm), Italy c CNR-Istituto di Struttura della Materia, via del Fosso del Cavaliere 100, I-00133 Roma, Italy

Abstract We have studied the introduction of deep levels in InGaAs/InP multi-quantum wells (MQW) grown by metalorganic vapor phase epitaxy using tert-butylarsine (tBuAsH2) as arsenic precursor as well as chromium (bis-cyclopentadienyl chromium) and zinc (diethyl-zinc) as dopant elements able to promote recombination centres in the ternary alloy. We have utilized secondary-ion mass spectrometry to measure dopant concentrations and high-resolution X-ray diffraction for structural characterization. Particularly for the chromium-doped systems, frequency- and time-resolved optical spectroscopy of the MWQ ground-state transition confirms an increase of the carrier recombination rate upon an increase of defect density, while the electronic and structural properties of the sample are largely preserved. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.
a; 68.55.Ce; 61.10.
i; 78.55.
Cr; 78.66.
Fd Keywords: A1. As antisite; A1. Chromium dopant; A1. Photoluminescence; A3. Metalorganic vapor phase epitaxy; A3. Multi quantum wells; B1. InP/InGaAs

1. Introduction A growing number of semiconductor devices rely on the ability to integrate optical switches. A means to improve the speed of an optical switch involves a careful and controlled introduction of defects that may act as efficient recombination centres. This is particularly true when excitonic recombination is used as the switching mechanism of the device. Several chemical elements are known to promote the formation of recombination *Corresponding author. Fax: +39-49-870-2911. E-mail address: [email protected] (G. Carta).

centres in gallium arsenide and related aluminium and indium alloys. These include: beryllium [1], iron [2], and arsenic overflux leading to the formation of As antisites [1,3]. In this paper, we present a study on the incorporation of deep Cr levels in the ternary alloy of lattice-matched InGaAs/InP multi-quantum well (MQW) samples by using bis-cyclopentadienyl chromium (Cp2Cr) as dopant precursor and tert-butylarsine (tBuAsH2) as arsenic precursor during metalorganic vapor phase epitaxy (MOVPE) growth. We have also analysed the possibility to promote deep levels by the use of zinc. Under standard growth condition, it is known that when gallium arsenide

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 8 5 1 - 1

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150

is doped with magnesium, it displays El2 electron recombination centres while zinc can introduce deep hole traps [4]. Owing to tBuAsH2’s higher thermal decomposition efficiency [5], we investigate whether it is possible to obtain El2 deep traps also for the case of zinc doping using this organometallic arsenic precursor. Moreover, theory suggests that zinc reduces the activation energy for the incorporation of an As atom into a Ga lattice centre [6], with a subsequent facilitation of As-antisite formation. The aim of this work is to assess the effects of the incorporation of these defects on the structural and optical properties of the aforementioned MQW systems, as well as to investigate which method between Zn doping or direct Cr incorporation provides the most efficient means of introducing non-radiative recombination channels.

2. Growth procedure and structural analysis Two series (hereafter series ‘‘A’’ and ‘‘B’’) of lattice-matched InGaAs/InP MQW heterostructures, with thirty 5.5 nm InGaAs wells and thirty 11 nm InP barrier layers have been grown on (0 0 1)70.51 oriented s.i. InP substrates in an AIXTRON AIX-200-system (ptot ¼ 5 kPa, Tg ¼ 6401C, growth rate=1.06 mm/h). The group-III sources were trimethylindium (TMIn, EPICHEM, 0.21 Pa) and trimethylgallium (TMGa, EPICHEM, 0.10 Pa); the group-V sources were phosphine (5.5 Phoenix, 113 Pa) and tBuAsH2 (MOCHEM). The use of tBuAsH2 is motivated by previous work where it is demonstrated that InGaAs/InP heterostructures grown in a MOVPE apparatus with such an As precursor show high structural and optical quality compared to those

grown with arsine, a more toxic and dangerous compound [5]. Growth interruptions were implemented to promote interface abruptness. In this way, we attempted to balance two opposing effects: (i) a migration and reorganization of chemisorbed species which favour surface flatness and (ii) a detrimental, partial surface decomposition that concurrently takes place [5]. For series A, Cp2Cr (EPICHEM) was used as the dopant source for the ternary alloy at different bath temperatures. For series B, the partial pressure of TBAs was doubled and a flux of DEZn (EPICHEM) was injected in the growth reactor at various partial pressures. Table 1 shows the growth parameters for the intrinsic samples corresponding to the two series (1A, 1B) as well as for two representative samples doped with chromium and zinc (2A and 2B, respectively). The column labelled V/III reports the ratio of partial pressures for group-V and group-III species. For the doped Cr and Zn samples, the elemental concentration n (as inferred from secondary-ion mass spectrometry) is reported. High-resolution X-ray diffraction (HRXRD) curves were measured with a Philips MRD diffractometer. All samples show narrow satellite peaks, the intrinsic MQW 1A showing the highest absolute diffraction intensity and sharpest satellite peaks toward higher orders. A best fit of the experimental data based on Vegard’s law was obtained by assuming the formation of compositionally graded interfaces with a convex exponential path. In this manner, one may account for both the carry-over of phosphorus into the InGaAs alloy at the lower interface (LI—InxGa1
xAsyP1
y layer) as well as the incorporation of As into the InP barrier at the upper interface (UI—InPyAs1
y layer). In Table 1 we report the thickness (th) of the graded layers

Table 1 Growth parameters and samples description. Symbols are described in the text Sample 1A 2A 1B 2B

Dop. source — T(Cp2Cr) 191C — DEZn 1.1  10
1 Pa

n (cm
3) — 1018 — 1018

V/III 5.2 5.2 10.3 10.3

LI th (nm) 1.1 1.1 1.1 1.1

LI da=a
3


3


8.3  10 /–1.7  10
7.6  10
3/0.3  10
3
7.6  10
3/–0.9  10
3
6.2  10
3/1.1  10
3

UI th (nm)

UI da=a

1.5 2.5 1.5 1.5

1  10
2/0 1.4  10
2/0 1.2  10
2/0 1.4  10
2/0

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and the lattice-mismatch interval (da=a) of the grading profiles obtained from the fits.

3. Optical characterization and discussion All MQW samples were characterized by means of reflectivity, transmittance, and photoluminescence (PL) over the temperature range T ¼ 102300 K. For the sample set shown in Table 1, we report in Fig. 1 the relative PL intensity as a function of sample temperature. In Fig. 2, the PL spectra at 10 K are also given. The experimental set-up and the procedure for data analysis have being described in previous works [5,7–9]. The intrinsic samples 1A and 1B show narrow PL peaks (B10 meV at T ¼ 10 K), a quasi constant PL intensity vs. temperature (cf. Fig. 1), negligible Stokes shifts (o1 meV) as well as sharp absorption edges. In going from the intrinsic samples to the doped ones, we observe a marked reduction in the absolute PL intensity at 10 K and a much stronger dependence with temperature. For those samples grown under a higher As overpressure, the quenching of the PL efficiency is proportional to the Zn concentration. In particular, for sample 2B, the effect amounts to one order of magnitude in going from T ¼ 10 K to room temperature. This effect is accompanied by a broadening of the low-temperature PL line shape

Fig. 1. PL intensity as a function of sample temperature for all MQW samples relevant to this study. Intensities are referred to the PL intensity of sample 1A at T ¼ 10 K.

Fig. 2. The figure shows the PL line shape at low temperature (T ¼ 10 K). For an easier reading of the figure, the PL spectra have been normalized to the same peak amplitude and shifted to a common peak energy.

by about a factor of two (cf. Fig. 2). Transmission measurements at room temperature show the same degradation of the optical transition. For the Crdoped sample (2A), the absolute PL efficiency at room temperature amounts to three orders of magnitude less than the efficiency of the intrinsic ones, with a drop of the PL efficiency with temperature of two orders of magnitude. At the same time, the sample shows a narrow, lowtemperature PL line shape with no Stokes shift and a sharp absorption edge, as previously observed for the intrinsic samples 1A and 1B. From the evolution of the PL intensity vs. temperature [10] we argue that non-radiative recombination channels are driven by phonon-like scattering processes with activation energies B21 meV and with a similar phonon-coupling efficiency for all samples. This implies that the sample-to-sample differences in non-radiative recombination efficiencies at high phonon densities (i.e., at room temperature) are mainly related to a different density of states of these recombination channels. To further check the optical quality of the Crdoped sample (2A), we also performed PL excitation correlation (PEC) spectroscopy [11–13] at low temperatures (To100 K) by exciting the MQW system below the InP barrier under low-power

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excitation conditions (B1011 photons/cm2 per pulse). Under these conditions, the asymptotic PEC decay constant corresponds to the effective (radiative and non-radiative) recombination time of the system [14]. We observed recombination lifetime ranging from 2 to 4 ns for 10 K oTo100 K. This feature indicates that the lowtemperature recombination kinetics in sample 2A is dominated by radiative decay [15]. That is to say, in the temperature regime where phonon scattering is not dominant, sample 2A behaves as a high-quality MQW, its optical quality not being affected by the introduction of Cr defects.

4. Conclusion HRXRD and extensive spectroscopic data show that Cr doping does not affect the crystalline quality, alloy homogeneity, and interface flatness of MOVPE-grown InGaAs/InP MQWs. Instead, the presence of this metal produces deep defects that accelerate the de-excitation of photogenerated carriers. The use of Zn under As overpressure yields not only a weaker temperature dependence of the PL intensity (i.e., slower recombination kinetics) but also a more clear degradation of the optical quality of the sample. These features indicate that Cr doping in MOVPE-grown InGaAs/InP MQWs constitutes a suitable approach to enhance the switching speed of devices based on these heterostructures.

Acknowledgements The authors are indebted with Mr. A. Camporese for the growth of the samples. Dr. F. Fernandez Alonso acknowledges support in the

form of a Marie Curie Fellowship of the European Program ‘‘Improving Human Research Potential and the Socio-economic Knowledge Base’’ under contract number HPMFCT-2000-00683.

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