† Corresponding author. E-mail:
Project supported by the the Science Challenge Project of China (Grant No. TZ2016003), the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, and 61474110), and Beijing Municipal Science and Technology Project (Grant No. Z161100002116037).
In this work, we study the influence of carrier gas H2 flow rate on the quality of p-type GaN grown and annealed at lower temperatures. It is found that the concentration of H atoms in Mg-doped GaN epilayer can effectively decrease with appropriately reducing the carrier gas H2 flow rate, and a high-quality p-type GaN layer could be obtained at a comparatively low annealing temperature by reducing the carrier gas H2 flow rate. Meanwhile, it is found that the intensity and wavelength of DAP peak are changed as the annealing temperature varies, which shows that the thermal annealing has a remarkable effect not only on the activation of acceptors but also on the compensation donors.
In recent years, gallium nitride (GaN) and its ternary alloys have attracted a great deal of attention thanks to their material properties, which are advantageous for applications in light emitters and detectors devices.[1–5] Nevertheless, the quality of p-type GaN sometimes still acts as a bottleneck that limits the performance of these devices.[6] Up to now, Mg, as a singly useful element, has been used to dope p-type GaN. The Mg atoms doped in the GaN layer are often passivated by H atoms through forming a neutral Mg–H complex because the H atoms are contained in the growth environment for metal organic chemical vapor deposition (MOCVD) system.[7,8] This issue was first treated by Amano et al. through the low-energy electron beam irradiation.[9] Unfortunately, this method has a fatal defect in that it only can activate several hundred nanometer-thick materials. Subsequently, thermal annealing reported by Nakamura et al. is successful to solve this Mg passivated problem.[7] Nevertheless, some recent studies found that the high growth temperature and annealing temperature for p-type GaN will lead to In-segregation in InGaN quantum wells (QWs) of the active region for laser diode (LD) or light emitting diode (LED).[10–13] Therefore, relatively low growth and annealing temperatures are needed to avoid reducing the device performance. However, it is often difficult to achieve high quality p-type GaN at lower growth temperature as the crystalline quality degenerates with the decreasing of growth temperature. The degeneration of crystalline quality means that the density of defects will be largely increased and deteriorate the quality of p-type GaN. At the same time, previous studies have proven that the efficiency of H atoms diffusing out of GaN layer will decrease with f annealing temperature lowering,[14] which will affect the efficiency of Mg acceptor activation. Therefore, to achieve a high-quality p-type GaN with a comparatively low growth and annealing temperature, some other growth conditions must be changed accordingly. In this paper, the influence of carrier gas H2 flow rate on the quality of lower-temperature-grown and-annealed p-type GaN is studied.
A series of 1-μm thickness Mg-doped GaN films was grown on a 2-μm-thick unintentionally doped GaN layer in an MOCVD system. Trimethylgallium (TMG), ammonia (NH3), and Bis-cyclopentadienyl magnesium (Cp2Mg) were used as precursors for Ga, N, and Mg, respectively. The H2 was taken as the carrier gas to transport all precursors into the MOCVD reactor. The detailed parameters for p-type layer growth are shown in Table
Figures
To obtain more information about these two samples, we have shown the Mg concentration profile obtained through SIMS measurement in Fig.
It is noticed that even the total Mg doping concentration in sample B is lower than that in sample A. In addition, the the p-type properties of sample B are much better than those of sample A at a comparatively low annealing temperature. To understand the Hall measurement results for these two samples annealed at different temperatures, we measure the H concentration profiles for these two samples after being annealed at different temperatures. Figure
Generally, we hold the view that the thermal annealing is a process of Mg activation, in which the Mg–H complexes are depassivated and H atoms are released away from the material. Some researches suggested that the temperature dependence of quality for p-type GaN in the annealing experiment comes from the different escape efficiencies of H atoms away from the epilayer.[14] Nevertheless, in our experiment we can find that the p-type quality has a big improvement with annealing temperature increasing from 550 °C to 750 °C while the net H concentration in either A or B almost keeps uninfluenced by the annealing temperature. The carriers’ concentration for sample A and sample B are 6.0 × 1016 cm−3 and 9.0 × 1016 cm−3 respectively when the annealing temperature increases from 550 °C to 750 °C. Obviously, the p-type quality improvement in our experiment cannot be explained by the model of escaping H atoms from the p-GaN epilayer.[14] The H atom in GaN material can exist in different states such as negative ion (H−), positive ion (H+), or neutral charge states (H0, H2). In p-type GaN most of H atoms are H+ ions before annealing the sample as H+ has the lowest energy state, and the H+ ion always forms a neutral complex with Mg acceptor in GaN and passivate it.[15–17] It is reported that a thermal annealing can make H atoms change into various states, and the concentration for each state is determined by the thermal equilibrium and affected by annealing temperature.[18] The improvement of p-type quality means that the H+ concentration in p-type layer decreases with annealing temperature increasing in our experiment. With the increase of annealing temperature, more and more Mg acceptors are activated by thermal annealing. It is worth noting that for all states of the H atom, only H2 can diffuse out to escape from the material. Consequently, our experimental results can be explained by the fact that at the relatively low annealing temperature between 550 °C–750 °C, even though the H atom state changes with the increase of annealing temperature, the H atoms’ concentration in p-type GaN layer keeps unchanged as the H2 formation efficiency is still quite low. This result is consistent with the theoretical calculation made by Myers et al. which shows that H2 forms only when the H atoms’ concentration begins to exceed uncompensated Mg concentration.[19] When the annealing temperature continually increases to 850 °C, the H2 formation efficiency is improved, which causes the H atoms’ concentration to decrease significantly, and this improves the p-type quality. From this analysis, we can conclude that the H atom’s concentration in the p-type GaN layer can be effectively reduced by appropriately reducing the flux of carrier gas H2, which is helpful in achieving better quality with a comparatively low annealing temperature. At the same time, we find that the Hall measurement result can be improved while the H atoms’ concentration in the material is almost the same after being annealed at a comparatively low temperature, which can happen due to the fact that at this annealing temperature, the state of H atom changes but the probability of forming H2 molecules is still low.
We also measure the room-temperature PL spectra for these samples as shown in Fig.
In this work, we investigated the effect of carrier gas H2 flux on the quality of lower temperature growth and annealed p-type GaN through Hall measurement, SIMS and room temperature PL. We find that the H atoms’ concentration in Mg-doped p-type GaN layer epilayers can effectively decrease as the flow rate of carrier gas H2 is appropriately reduced In addition, a high-quality p-type GaN can be obtained at a comparatively low annealing temperature. Meanwhile, we find that when annealing the sample at a temperature in a range of 550 °C–750 °C, the Hall measurement result can be improved while the H atoms’ concentration in the layer is almost the same after being annealed in the temperature range. This can happen because in this annealing temperature range, the state of H atom changes but the probability of forming H2 molecules is still low. Finally, we find that the thermal annealing has a remarkable effect, not only on the activation of acceptors but also on the concentration of compensation donors.
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