Growth Rate Reduction of GaN Due to Ga Surface Accumulation
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 12
Growth Rate Reduction of GaN Due to Ga Surface Accumulation D.E. Crawford, R. Held, A. M. Johnston, A. M. Dabiran, Philip I. Cohen Department of Electrical Engineering, University of Minnesota This article was received on July 1, 1996 and accepted on September 23, 1996.
Abstract GaN(0001) has been grown on Al2O3 (0001) by molecular beam epitaxy where NH3 was used as the nitrogen precursor. Desorption mass spectroscopy and reflection high energy electron diffraction (RHEED) were used to monitor the relationship between growth rate and the incident fluxes during growth. Excess surface Ga decreases the GaN formation rate when the substrate temperature is too low or the Ga flux is too high. A simple rate equation is used to describe the observed behavior.
1. Introduction A number of issues related to growth kinetics of GaN must be addressed. Historically, the n-type conductivity commonly exhibited by GaN was attributed to N vacancies [1]. This problem was thought to arise from a kinetic barrier to N incorporation during growth. Recent results by Lee et. al [2] and also by Jones et. al [3] using NH3 as the nitrogen precursor during molecular beam epitaxy (MBE) indicate that the GaN growth rate is strongly temperature dependent, exhibiting a maximum between 750°C and 800°C. The high temperature decrease in growth rate is typical of GaN, and has been attributed to Ga desorption at elevated substrate temperature [2] [3] as well as to the decomposition of GaN [4]. At lower substrate temperature the common belief is that reduced N incorporation efficiency limits the growth rate [2]. In this paper we will address the low temperature issues related to GaN MBE where the GaN decomposition rate is much less than the growth rate . In section 3.1 we examine the transient response of the GaN surface composition and chemistry to a step-function of Ga flux. The time dependence of the Ga desorption after exposure of the surface to incident Ga indicates that Ga adsorbs to the surface in two states. The first deposited layer of Ga sticks to the surface in a strongly bound state, whereas subsequent desorption occurs at a higher rate. The more tightly bound of these layers is observed only when the surface has been previously exposed to NH3. In section 3.2 we examine the relationship between growth rate, substrate temperature and the incident fluxes. We show how desorption mass spectroscopy (DMS) can be used to determine whether the GaN formation rate is limited by the available N (Ga rich) or by the available Ga (N rich) and present a single rate equation to explain a decrease in the GaN formation rate with increasing Ga flux.
2. Experimental Growth was carried out in a cryopumped Gen II MBE system which is shown schematically in Figure 1. A quadrupole mass spectrometer mounted in one of the source ports enabled detection of the type and intensity of desorbed species from the substrate surface. The specular RHEED intensity was monitored using a photomultiplier
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