Aluminum–gallium–nitride (AlGaN) alloy is recognized as one of most promising materials for applications in deep ultraviolet (DUV) light emitting diode (LED) devices,^[1–3] due to their great potential applications in sterilization, water purification, and biological and chemical agent detection.^[4,5] In order to fabricate high efficiency DUV-LEDs, it is required to obtain a highly conductive p-type AlGaN layer with a high AlN mole fraction. However, since the ionization energy for Mg-doped Al_xGa_{1 − x}N alloys dramatically increases with AlN mole fraction increasing from 170 meV (for x = 0) to 630 meV (for x = 1),^[6,7] which leads to very small hole concentration in the valence band at room-temperature. Therefore it is rather difficult to achieve highly conductive p-type AlGaN.

Recently, very high Mg-doped level (around 10²⁰ cm⁻³–10²¹ cm⁻³) could be adopted to obtain adequate free hole conductivity, because of the low activation ratio of holes in AlGaN with high AlN mole fraction.^[8] But high doping effect easily leads to the valence band-tail states and impurity band to form.^[9] In addition, since the formation energy values of intrinsic defects and their complexes are significantly reduced in highly p-type Al-rich AlGaN alloys, these defects could further compensate for the presence of free holes.^[10,11] So the fundamental understanding of the impurity transitions, as well as Mg acceptor activation mechanism and hole transport characteristics in highly Mg-doped AlGaN has become increasingly important.

In this paper, we systematically investigate the optical transitions of impurity transitions and variable-temperature Hall effect to clarify the Mg acceptor activation mechanism and transport characteristics in highly Mg-doped AlGaN alloys. The optical characteristics of undoped AlGaN and Mg-doped AlGaN alloys are investigated by using the cathodoluminescence (CL). The electrical properties of each sample are analyzed by using the temperature-dependent Hall effect measurements. It is discovered that there exists a pronounced reduction activation energy for Mg acceptor and lower hole mobility in highly Mg-doped AlGaN alloys, due to the high ionized acceptor concentration and compensation ratio.

The unintentionally doped and Mg-doped AlGaN alloys were grown by metal–organic chemical vapor deposition (MOCVD) using trimethylaluminum (TMAl), trimethylgallium (TMGa), ammonia (NH₃) as the organometallic precursors for Al, Ga, and N, respectively. Hydrogen (H₂) was used as the carrier gas. For Mg doping, an optimized molar flux of biscyclopentadienyl-magnessium (Cp₂Mg) at 4.8 × 10⁻⁷ mol/min was used. More detailed growth procedures for undoped and Mg-doped Al_xGa_{1 − x}N alloys were reported in Refs. [12,13]. The AlN mole fractions of AlGaN samples were determined by high-resolution x-ray diffraction (HRXRD) through using Cu K line = 0.15406 nm radiation (PANalytical X’Pert Pro XRD). The concentration of Mg atoms incorporated into Mg-doped AlGaN alloys was determined to be 1 × 10²⁰ cm⁻³ by using secondary ion mass spectrometry (SIMS).^[13] The optical properties were characterized by cathodo-luminescence spectra (CL, Gatan Mono CL3+) at low temperature ∼ 120 K. Post growth rapid thermal annealing in N₂ ambient was used to activate Mg acceptor. The electrical properties were measured by variable-temperature Hall effect measurement through using the van der Pauw method (Accent HL5500 measurement system).

Figure 1 shows the comparison between the CL spectra measured at the low temperature (120 K) for undoped and highly Mg-doped Al_xGa_{1 − x}N alloys of varying x (0.23, 0.35, and 0.57). The band edge transitions are at 3.955 eV, 4.161 eV, and 4.566 eV for the undoped Al_xGa_{1 − x}N (x = 0.23, 0.35, and 0.57) alloys, respectively, due to the free excition (FX) transition.^[14] In additon, it is observed that the peak energy position of impurity transition, denoted as (DAP)_I shifts from 3.642 eV for x = 0.23 to 4.146 eV for x = 0.57, since the Si, O, and C impurities are easily incorporated during MOCVD growth.^[15]

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. Low-temperature (120 K) CL spectra of undoped and highly Mg-doped AlxGa1 − xN alloys with varying x (0.23, 0.35, and 0.57) with dashed lines and solid lines denoting undoped and Mg-doped samples, respectively.

Figure 2(a) shows the plots of the CL spectral peak position (E_emi) versus AlN mole fraction (x) of the above impurity transitions in undoped AlGaN alloys together with those of cation vacancy complexes with one-negative charge (V_III-complex)⁻¹ related transitions. The energy position of these impurity transitions show a continuous increase with AlN mole fraction increasing. Correlating the onset of much oxygen concentration in SIMS results,^[13] we suggest that the presently observed impurity transition shown in undoped AlGaN alloys is also attributed to the recombination between electrons bound to oxygen donors and (V_III-complex)⁻¹, such as (V_Ga-2O_N)¹⁻.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (a) CL peak position (Eemi) of present impurity transition (solid stars) observed from Fig. 1 and those of (VIII complex)1− related transitions (solid squars) obtained from undoped AlxGa1 − xN alloys as a function of AlN mole fraction x in Ref. [16]; (b) EA versus AlN mole fraction for (VIII-complex)1−, Ev, Ec, and ED.

To obtain a more comprehensive picture of O donor and (V_III-complex)⁻¹ acceptor energy levels in undoped AlGaN alloy, we plot its donor, acceptor energy levels, conduction (E_c), and valence (E_v) band edges versus AlN mole fraction x in Fig. 2(b). Now, the ionization energy value of the O donor (E_D) is assumed to increase linearly from 25 meV to 86 meV with x increasing from 0 to 1 in Al_xGa_{1 − x}N alloys,^[17,18] and the peak position of band edge transition for undoped AlGaN alloy is considered, other parameters and the same procedure are used as described in Ref. [16]. The Coulomb interaction between the ionized donors and acceptors is neglected. The acceptor level (E_A) in undoped Al_xGa_{1 − x}N alloys as a function of AlN mole fraction x could be written as

For Mg-doped AlGaN alloys, the band edge transitions become completely quenched asindicated by the solid line of Fig. 1. Compared with those in undoped samples, (DAP)_I in Mg-doped samples has a slightly lower energy position, which is usually ascribed to the fact that the Mg incorporation makes the magnitude of the strain changed slightly.^[19] The emission lines at 3.263 eV, 3.262 eV, and 3.589 eV are dominant for Mg-doped Al_xGa_{1 − x}N alloys (x = 0.23, 0.35, and 0.57), respectively, which do not occur in undoped AlGaN alloys. So those emissions are attributed to Mg impurity-related transitions. Since the formation energy of the nitrogen vacancy with three positive charges () is small, it is suggested that the positive charges () be the most dominant compensating centers in Mg-doped Al_xGa_{1 − x}N alloys.^[6,10] Therefore, we suggest that the physical origin of those emission lines is consistent with that of the 2.8-eV line in Mg-doped GaN^[20–25] and the 4.7-eV line in Mg-doped AlN,^[6,26] which is ascribed to the recombination of electrons bound to and neutral Mg acceptors, denoted as (DAP)_II.^[27] Similarly, we further investigate the energy levels (E_A, E_D) of acceptor and donor with conduction and valence band edge (E_c, E_v) versus AlN mole fraction (x) in Mg-doped Al_xGa_{1 − x}N alloys as shown in Fig. 3. The spectral peak position (E_emi) of impurity transition, together with 2.8-eV line in Mg-doped GaN and 4.7-eV line in Mg-doped AlN is also plotted for Al_xGa_{1 − x}N alloys as a function of x. Based on the Hall effect measurement results and reported Mg acceptor levels,^[6] the energy levels have been deduced in Mg-doped Al_xGa_{1 − x}N in the whole range of AlN mole fraction.^[27]

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Plots of energy levels (EA, ED) of acceptor and donor with conduction and valence band edge (Ec, Ev) versus AlN mole fraction (x) in Mg-doped AlxGa1 − xN alloys.

In order to characterize the ionization of Mg acceptor, we carry out the temperature-dependent Hall effect measurements. The resistivity for Mg-doped Al_0.57Ga_0.43N alloy is highly resistive (not shown), which is most likely to be due to the resistivity exponentially increasing with the activation energy (E_A), and being compensated for by nitrogen vacancies (as shown in Fig. 1). So the temperature range where Hall measurements are performed is not large enough to determine E_A for Mg-doped Al_0.57Ga_0.43N alloy. Figure 4(a) shows the temperature-depenmdent resistivity curves of Mg-doped Al_xGa_{1 − x}N (x = 0.23 and 0.35) alloys. In Mg-doped Al_0.23Ga_0.77N alloys, the resistivity is almost constant at temperatures ranging from 200 K to 325 K, then it decreases with the temperature increasing from 325 K to 650 K, which clearly indicates that there exist two different hole transport mechanisms. However, the resistivity decreases exponentially by more than one order of magnitude in Mg-doped Al_0.35Ga_0.65N alloys, while the temperature is increased from 260 K to 535 K.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. Temperature-dependent (a) resistivity, (b) hole concentration, and (c) mobility of Mg-doped AlxGa1 − xN (x = 0.23 and 0.35). Solid lines are obtained from Arrhenius plots in high-temperature region.

Since the resistivity includes the contributions from both hole concentration (p) and mobility (μ), we evaluate the E_A values of Mg acceptors for Mg-doped Al_xGa_{1 − x}N (x = 0.23 and 0.35) based on the temperature-dependent free hole concentration as shown in Fig. 4(b). In Mg-doped Al_0.23Ga_0.77N alloys, the hole concentration exhibits an increasing trend with temperature increasing. But the hole concentration is insensitive to temperature in a lower temperature range from 200 K to 325 K, and E_A calculated from Arrhenius plots is 17 meV, indicating the onset of impurity band conduction.^[8,28,29] Thus, holes need not to occupy valence band states for carrier transport.^[30]

For the Mg-doped Al_0.35Ga_0.65N alloys, the hole concentration also increases with temperature increasing due to the thermal ionization of Mg dopants. There exists a deviation between the experimental measurement data and the fitting curve at temperatures ranging from 260 K to 360 K, which implies that the thermal activation of holes starts to play a role. But only temperature above 360 K, the thermal activation becomes dominant for the hole transport mechanism. The fitting lines are obtained by the following Eqs. (2) and (3) based on the charge neutrality condition

Table 1.

Table 1.

Table 1. Fitting results from variable Hall measurements of the Mg-doped AlxGa1 − xN (x = 0.23 and 0.35) alloys. . AlN mole fraction NA/cm−3 ND/cm−3 Compensition ratio/% EA/meV ΔEA/meV EA0/meV 0.23 4.45 × 1020 1.42 × 1019 3.2% 172 79 251 0.35 1.68 × 1020 6.89 × 1018 4.1% 242 63 305

Table 1. Fitting results from variable Hall measurements of the Mg-doped AlxGa1 − xN (x = 0.23 and 0.35) alloys. .

The values of activation energy (E_A) are 172 meV and 242 meV for Mg-doped Al_0.23Ga_0.77N, and Al_0.35Ga_0.65N, respectively, which are lower than the previously estimated value of of E_A as a function of the AlN mole fraction in Mg-doped AlGaN alloys.^[6] The lowering of the value of the activation energy (ΔE_A) is ascribed to the combined effect of the Coulomb potentials of the Mg dopants and the screening of the Coulomb potentials by a high concentration of free holes. Moreover, ΔE_A is assumed to be proportional to the average distance between ionized acceptors.^[32]

Thus, the acceptor activation energy may be expressed as

In this work, the optical and electrical characteristics of highly Mg-doped Al_xGa_{1 − x}N alloys (0.23 ≤ x ≤ 0.57) are investigated. Their cathodo-luminescence spectra show that there exist two groups of impurity transitions in Mg-doped AlGaN alloys: one is assigned to the recombination between electrons bound to shallow oxygen donors and (V_III-complex)¹⁻, and the other is the recombination of electrons bound between and neutral Mg acceptors, which indicates that there exists the obvious self-compensation phenomenon in Mg-doped AlGaN alloy. The values of effective acceptor activation energy (E_A) in Mg-doped Al_xGa_{1 − x}N films (x = 0.23 and 0.35) are determined to be 172 meV and 242 meV, and the hole concentrations are 1.2 × 10¹⁸ cm⁻³ and 3.3 × 10¹⁷ cm⁻³ at room temperature, respectively. It is important to consider the Coulomb interaction between ionized acceptors, which reduces the activation energy due to increasing the degree of ionization and compensation ratio. Furthermore, the other high doping effects are also observed, including impurity conduction and lower hole mobility.