H. Heinecke

Selective area growth of III/V materials in metalorganic molecular beam epitaxy (chemical beam epitaxy)

Abstract This paper describes the current status in selective area epitaxy using masked wafers in metalorganic molecular beam epitaxy (MOMBE, or chemical beam epitaxy, CBE). The surface selective growth is discussed leading to a better understanding of the growth phenomena taking place in the transition areas from growth to non-growth where crystal facets are formed. Based on this knowledge it is demonstrated that unique structures with uniform layer thickness and material compositions can be obtained, not depending on the aspect ratio of the masked area. This enables for the device designer a flexible mask layout not limited due to the growth mechanism. Finally the current knowledge on selective area growth (SAE) of device structures is highlighted by examples like monolithic integration of laser waveguides.

Beam geometrical effects on planar selective area epitaxy of InPGaInAs heterostructures

This communication reports on the lateral growth at planar selective area epitaxy (SAE) grown vertical sidewall ridge structures. This lateral growth is investigated as a function of the angle of the molecular beam with respect to the substrate surface, the width of the mask and the ridge size. The results prove that for molecular beams arriving perpendicularly to the substrate surface the lateral growth is about half compared to the conventional geometry. This enables the planar embedded growth of heterostructures as used in integrated device approaches using identical growth parameters as in the planar SAE.

Growth of GaInAs(P) using a multiwafer MOMBE

We have studied the uniform growth of GaInAs(P) on InP using 5 inch substrate holders, designed for 2 inch single wafers to 3 × 2 inch multiwafer growth. The design of the holder, and thus the temperature gradient across the growth area shows a significant effect on the compositional distribution. Design rules and an automated surface temperature control are discussed. We obtain GaInAs layers with a total variation in lattice mismatch of less than 330 ppm along a 46 mm wafer diameter. This is almost independent of beam geometry. Quaternary layers with λ = 1.558 μm exhibit a total variation in lattice mismatch of less than 190 ppm along a 46 mm wafer diameter. The shift in a 300 K PL wavelength is below the resolution limit of 1 nm across the entire wafer area. Using eccentric 3 × 2 inch multiwafer geometry we obtain GaInAs with a total variation of 900 ppm along the wafer diameter in the radial substrate holder direction and 350 ppm from centre to edge perpendicular to this direction.

Evaluation of cracking efficiency of As and P precursors

The cracking behaviour of PH 3 , TBP and DitBuPH, AsH 3 and TBAs was studied by mass spectrometry in a production-type MOMBE growth chamber. The measurements were performed varying the cracking temperature of the injector cells and the total material flux. The pyrolysis of the hydrides PH 3 /AsH 3 essentially leads to the production of hydrogen and the dimers P 2 /As 2 . The decomposition of these source materials reaches about 90% at beam pressures in the range of 10 -7 Torr. Under identical conditions TBP, TBAs and DitBuPH are cracked better than 97%. However, the pyrolysis of the alkyl-compounds is accompanied by the formation of isobutene and hydrides. Consequently the decomposition of this PH 3 and AsH 3 produced during cracking is limiting the growth relevant cracking efficiency to the values obtained for the hydrides. The additional occurrence of dihydrides at beam pressures in the range of 10 -5 Torr suggests an initial decomposition process of the source materials via a radical formation mechanism. With respect to the stability of the compounds a self-dissociation of TBAs in the bubbler under the production of isobutane C 4 H 10 and AsH 3 was detected.

GRINSCH GaInAsP MQW laser structures grown by MOMBE

For applications in long wavelength MQW lasers, GaInAsP/InP heterostructures were grown by metalorganic molecular beam epitaxy (MOMBE or CBE). The growth process was performed with all gaseous sources for group III, group V and dopant precursors. In addition to the standard strained MQW laser structure with two quaternary separate confinement layers on each side of the active MQW region, laser structures with continuously graded GaInAsP confinement layers (GRINSCH) were prepared. In the latter case all group III and group V flows were ramped synchronously while maintaining lattice matching growth conditions. By this method a parabolic variation of the band gap was obtained. Data from first test lasers are comparable to those from standard lasers revealing the high material quality of MOMBE grown InP-based GRINSCH structures. Besides standard strained quaternary layers InAsP layers with compressive strain up to 2% were used as quantum wells, and InAsP MQW lasers with emission wavelengths of both 1.3 and 1.55 μm were fabricated. Moreover, an excellent MQW wavelength uniformity (standard deviation Δλ ≤ 2 nm) across areas larger than 95% of a 2 wafer along with proven thermal stability demonstrates the high yield of the process for all structures revealing the feasibility of this material for industrial use.

Lateral coupling of InP/GaInAsP/InP structures by selective area MOMBE

Lateral couplings of InP/GaInAsP/InP structures selectively grown by metalorganic molecular beam epitaxy (MOMBE) are presented. The heterostructures were grown by either using the hydrides AsH3 and PH3 or tertiarybutylphosphine and tertiarybutylarsine as group V precursors in a prototype multiwafer (MOMBE) system. The base heterostructures of the first epitaxy were patterned with SiO 2 mask stripes and trenches were reactive-ion etched. Heterostructures were selectively filled in by a second growth run forming lateral heterojunctions of different quaternary materials. The structures exhibit a bright photoluminescence and a sharp transition at the boundary of the locally grown material indicating a high crystal quality up to the lateral junction with only a minor change in the PL wavelength ( < 2 nm). These optimized lateral couplings were applied to laser/waveguide butt-couplings. Optical coupling losses were determined by means of reactive-ion etched waveguides across cascades of coupled GaInAsP layers with an emission wavelength of λ G 1050 nm. Values as low as 0.12 dB/coupling were determined in cut-back measurements.

Zn doping of InPGaInAsP device structures in metalorganic molecular beam epitaxy using diethylzinc

Diethylzinc was used as a gaseous p-type dopant source for growth of InP/GaInAsP layers in metalorganic molecular beam epitaxy. In InP layers a significant effect of growth temperature on Zn incorporation and on electrical activation has been found. Additionally, data from a variation of the dopant cracker cell temperature suggest that the dopant molecules should not be fully decomposed for an efficient dopant incorporation. A comparison of Hall data with data from secondary ion mass spectrometry (SIMS) reveals that in InP up to 60% of the acceptors appear to be electrically active under optimized experimental parameters, in GaInAs the activation is above 90%. The SIMS data show that dopant profiles with steep flanks can be obtained in InP/GaInAsP structures. However, a dopant redistribution occurs, which is more pronounced in InP layers. This effect is correlated with the dopant incorporation behaviour on substitutional and interstitial sites. The dopant incorporation process is discussed in detail, and the implications for growth of InP/GaInAsP device structures are outlined.

Planar selective area growth of DH laser structures using hydrides, tertiarybutyl and ditertiarybutyl group V precursors in MOMBE

In Metal Organic Molecular Beam Epitaxy (MOMBE) the selective area epitaxy (SAE) of double heterostructures (DH) lasers was investigated by using as group V starting materials tertiarybutylarsine/tertiarybutylphosphine (TBAs/TBP), ditertiarybutylarsine/ditertiarybutylphosphine (DTBAs/DTBP) and AsH 3 /PH 3 . In Zn-doped InP layers a reduced doping efficiency was found for the process using organic group V precursors whose dissociation chemistry can explain this observation. The growth of GaInAsP close to InP (Q 1.05) using TBAs/TBP reveals that there is a distinct change in the elemental incorporation compared to the hydride process. Thus individual tuning of the growth parameters for different precursors is required. The selectively grown laser ridge structures compare well with large-area grown reference structures. The shape of the ridge is perfectly rectangular when Be-doped p-type cladding layers are used, whereas Zn doping yields a slight multifacet formation at the ridge sidewalls.

Growth of 1.55 μm DH laserstructures using TBAs and TBP in MOMBE

In a comparative study we have chosen TBAs and TBP as well as AsH 3 and PH 3 for the growth of InP/GaInAs(P) heterostructures for laser applications in a production metalorganic molecular beam epitaxy (MOMBE) system. The n-type doping was performed with Si from an effusion cell, whereas for the p-type doping Be and DEZn were utilized. InP layers using TBP under optimized cracking conditions exhibit excellent surface morphology with good electrical properties in the low 10 15 cm -3 range of carrier concentrations. The MOMBE growth mechanism is not disturbed by the hydride replacement compound. This allows for a convenient replacement without losing calibration data from the hydride process. Broad-area DH laserstructures with GaInAsP (λ = 1.55 μm) active regions were grown with AsH 3 /PH 3 and TBAs/TBP. Comparable threshold current densities in the range of 1.6-2.3 kA/cm 2 are achieved for the lasers, grown with both sets of precursors combined with DEZn source doping. These results are in good agreement with the standard set by the hydride MOVPE process.

Element incorporation in GaInAsP for uniform large area MOMBE

In this study we investigated the material incorporation efficiencies in GaInAsP and InAsP layers grown on InP substrates for large area metalorganic molecular-beam epitaxy (MOMBE). We found an optimum growth temperature for the quaternary material (λ G = 1.55 μm) around 500-510°C, since in this range the lattice matching shows a temperature coefficient of about 80-120 ppm only. The P incorporation efficiency is improved with increasing growth temperatures (480-520°C). We found for InAsP and GaInASP (λ G = 1.05 μm) a strong reduction of the P incorporation efficiency with increasing V/III ratio, which is accompanied by a reduced P content in the layer. Additionally with this reduced P, the Ga incorporation efficiency for the quaternary material is improved. Using the same V/III and As/P ratio for the InAsP and GaInAsP material, a higher P content in the InAsP layers is achieved. The results were used as calibration data for the development of a novel type of wafer holder with improved temperature uniformity. For GaInAsP single layers (λ G = 1.55 μm ) a standard deviation in emission wavelength of |Δλ| < 1.5 nm was achieved. From the results for all material compositions a temperature distribution with AT < 1°C can be inferred across a 2 wafer.