The growth rate evolution versus substrate temperature and V/III ratio during GaN MBE using ammonia
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Internet Journal Nitride Semiconductor Research
The growth rate evolution versus substrate temperature and V/III ratio during GaN MBE using ammonia A. N. Alexeev1, B. A. Borisov1, V. P. Chaly1, D. M. Demidov1, A. L. Dudin1, D. M. Krasovitsky1, Yu. V. Pogorelsky1, A. P. Shkurko1, I. A. Sokolov1, M. V. Stepanov1 and A. L. Ter-Martirosyan1 1ATC
Semiconductor Devices Ltd.,
(Received Tuesday, May 4, 1999; accepted Wednesday, July 21, 1999)
The growth rate evolution versus V/III ratio and substrate temperature was studied by means of optical reflectivity during MBE of GaN layers using NH3 as nitrogen source. The GaN desorption becomes observable at temperatures above 800°C and causes the reduction of growth rate accompanied with the surface roughening at temperatures above 850-870°C. Unlike GaAs, which evaporates in accordance with the action mass law, the desorption rate of GaN is found to be almost independent of V/III ratio within the N-rich growth conditions. The activation energy for GaN desorption during the growth is found to be (3.2±0.1) eV. This value is very close to the activation energy for free evaporation. At V/III ratio values exceeding 200 the GaN growth rate reduction caused by violation of the molecular flow regime is observed. The Mg-doped samples grown under these extreme conditions tend to have improved acceptor activation and thus p-type conductivity.
1
Introduction
The group III nitrides are the subject of permanent interest during last decade due to their outstanding properties for optoelectronics. The current progress in this field is based on development of heterostructure growth techniques such as MOCVD [1] [2] and MBE [2] [3] [4]. The latter, however, remains rather sophisticated in literature due to a number of difficulties concerning the problem of proper active nitrogen sources, lower deposition temperatures, doping limitations etc.. The concept of deposition at higher V/III ratios, recently developed for an ammonia based process [5], seems to be very promising with respect to improvement of GaN structural and electrical properties. It is believed that both V/ III ratio and deposition temperature (Ts) are the key kinetic parameters to obtaining nitride layers of proper quality by MBE. Dependencies of the growth rate on these parameters are also very important in determining the upper temperature limit for MBE growth, since GaN thermally decomposes in high vacuum at lower temperatures than in most MOCVD systems. Held et.al. [3] have proposed the idea that GaN desorption under growth conditions should obey the action mass law, but there were no experimental data concerning that point. Grandjean et.al. [6] have recently reported experimental data indicating that GaN evaporation under NH3 flux
deviates from the thermodynamic behavior, but experiments under growth conditions were not carried out in this work. Here we report on results of the study of GaN growth kinetics by MBE using NH3 cracking. The desorption rate of GaN during the growth was in situ measured by the optical reflectivit
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