**2.2.2 Double carrier pulse DLTS**

492 Solar Cells – New Aspects and Solutions

denote the junction capacitance at reverse bias, the density of filled traps under steady state conditions, the ionized donor concentration, and the time constant that gives the emission

*<sup>N</sup> C C*

In most cases of using transient capacitance, the trap centers form only a small fraction of the SCR impurity density, i.e., *N*T << *N*D. Hence, using a first-order expansion of Eq. (2)

Thus, the trap concentration calculates from the capacitance change C is expressed by

*Tadj D C W N N*

*D*

*E E e vN*

*E E e vN*

where *n, N*c, and *vthn* are the thermal capture cross section, the density of states, and the thermal velocity of holes, respectively. *p, Nv*, and *vthp* are the same parameters for holes. *ECBM*, *EVBM*, and *ET* are the energy levels of the conduction band minimum, the valence band

The isothermal capacitance transient spectroscopy (ICTS) and the double carrier pulse DLTS (DC-DLTS) are two DLTS related methods. They are used to obtain the density profiling of lattice defects and to check whether they act as recombination centers or not, respectively.

 <sup>0</sup> 1 2 2 2

*e N* 

where *W*p, *E*F, and *E*T denote the SCR at *V*p, the Fermi level, and the trap energy level.

where *W*R is the total SCR at reverse bias voltage *V*R, *L*1 = *W*R - , *L*2 = *W*p - , and

The emission rates for electrons and holes are given, respectively by

*n n thn c*

*p p thp v*

**2.1.2 Thermal emission of carriers from deep levels** 

maximum, and the trap, respectively.

**2.2 Other DLTS related techniques** 

*T D <sup>C</sup> N N C* 

Note that Eq. (3) assumes that NT << ND and the traps are filled throughout the total depletion width. To be more accurate, NT should be adjusted to NTadj according to [30]

0

*C LL* 

*F T*

*E E*

*R*

*CBM T*

*kT*

*T VBM*

*KT*

2 2 2 0 12

*T D*

0 1 (2)

2 (4)

2 (5)

( ( )) (6)

exp (7)

exp (8)

*C C N N CN N* 0 0 *TD TD* 12 2 (3)

*N*

rate, respectively. The change in capacitance after the recharging of traps is given by

gives

DC-DLTS is used in asymmetric *n+-p* or *p+-n* junctions (Khan et al., 2005). It aims to check whether a trap is a recombination center or not. As shown in Fig. 1, two pulsed biases are applied to the sample, in turn, to inject majority and minority carriers to an electron trap. At the initial state, the junction is under reverse bias, and the energy level *E*T of the trap is higher than the Fermi level (*E*Fn).When the first pulse voltage is applied to the sample, *E*Fn is higher than *E*T, which allows the trap to capture electrons. During the second reverse biased pulse, with a duration *tip*, holes are injected to the SCR from the *p*side of the junction. After the junction pulse is turned off, electrons and holes are thermally emitted. The amount of trapped carriers can be observed as a change in the DLTS peak height of the trap. If the trap captures both electrons and holes, the DLTS maximum of the corresponding level decreases compared with that in conventional DLTS. Such a decrease is explained by the electronhole (*eh*) recombination process, which indicates that the level is a recombination center.

Fig. 1. Basic concept of capture and thermal emission processes from an electron trap located at an energy level *E*T in *p+-n* junction. A saturating injection pulse is applied to the reverse biased junction to fill the trap with holes.

Investigation of Lattice Defects in GaAsN

**4. Lattice defects in GaAsN grown by CBE** 

**4.1 Electron traps in GaAsN grown by CBE** 

**100 150 200 250 300**

**100 150 200 250 300**

*E***3**

**Tempertaure (K)**

**Temperature (K)**

**Annealed**

**x3**

*E***3**

**0.0**

**0.00**

**0.05**

**0.10**

**DLTS Signal (arb. unit)**

spectra.

**0.15**

*E***1**

**(c)**

**3.4**

**DLTS signal (arb. unit)**

**6.8**

**(a)**

grown by CBE will be dressed using DLTS and related methods.

**4.1.1 DLTS spectra and properties of a N-related electron trap** 

*E***2**

**Annealed**

*n*A to 10

Arrhenius plot, respectively.

Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy 495

*DL8000* was used for DLTS and C-V measurements. The activation energy *Et* and the capture cross section *n,p* were determined from the slope and the intercept values of the

In this section, the distribution of electron and hole traps in the depletion region of GaAsN

The DLTS spectrum of Fig. 2(a) shows an electron trap (*E*2) at 0.69 eV below the CBM of GaAs. After rapid thermal annealing at 720C for 2 min, *E*2 disappears completely whereas a new electron trap (*E*3) appears at 0.34 eV below the CBM. From the Arrhenius plots of Fig. 2(d), the capture cross sections of *E*2 and *E*3 are calculated to be E2 = 8.1 × 10-15 cm2 and E3 = 7.5 × 10-18 cm2, respectively. Based on previous results about native defects in *n*-type GaAs, *E*2 and *E*3 are independent of *N* and considered to be identical to *EL*2 and *EL*3, respectively (Reddy et al., 1996). In order to focus only on *N*-related lattice defects, these two energy levels will be excluded from the DLTS spectra of *N*containing *n*-type GaAsN. The addition

**100 150 200 250 300**

*E***1**

*<sup>E</sup>***<sup>2</sup> (d)**

**Tempertaure (K)**

*E***3**

**3.6 5.4 7.2**

**1000/T (1/K)**

**As grown**

*E***1**

*E***2**

**0.0**

**54.6**

**56.7**

**Ln.vth.Nc**)

**58.8**

Fig. 2. DLTS spectra of (a) *N* free as grown and annealed GaAs, (b) as grown n-type GaAs0.998N0.002, (c) annealed n-type GaAs0.998N0.002, and (d) Arrhenius plots of DLTS

**0.1**

**0.2**

**0.3**

**DLTS Signal (arb. unit)**

**0.4**

**0.5**

**(b)**

A for a maximum reverse bias voltage of -4 V. A digital DLTS system *Bio-Rad*

To formulate the recombination process, we consider the same notation in § 2.1.2, with assuming that *n ND*. The relationship between the total density of recombination centers and that only occupied by electrons in the n-side of the junction can be expressed by

$$\frac{dn\_{\rm T}}{dt} = -e\_{\rm p}n\_{\rm \tau}(t) + \langle p \rangle c\_{\rm p} [N\_{\rm \tau} - n\_{\rm \tau}(t)] - N\_{\rm D}c\_{\rm n}n\_{\rm \tau}(t) + e\_{\rm \tau}[N\_{\rm \tau} - n\_{\rm \tau}(t)] \tag{10}$$

where *p* is the average of injected holes. As a solution of Eq. (10), we have

$$\frac{dn\_{\tau}}{dt} = n\_{\tau}(t\_{\bar{v}}) = n\_{\tau}(\infty) + [n\_{\tau}(0) \cdot n\_{\tau}(\infty)] \exp(-\frac{t\_{\bar{v}}}{\tau}) \tag{11}$$

where *n*T() = (*pc*p+ *e*n)/( -1*N*T), -1 = *pc*p+ *N*D*c*n + *e*n + *e*p), and *tip* is the width of the injected pulse. Considering the IDLTS and IDC-DLTS the peak heights of the recombination center in conventional and DC-DLTS, respectively. Equation (11) can be rewritten properly as

$$\frac{m\_r \left(t\_{\rm ip}\right)}{N\_r} = \frac{\left(I\_{\rm DLS} - I\_{\rm DCDLS}\right)}{I\_{\rm DCDLS}} = \langle p \rangle \sigma\_p \upsilon\_{\rm tcp} t\_{\rm ip} \tag{12}$$

Similarly for hole trap in the *p*-side of a *n+-p* junction, which acts as recombination center, we obtain

$$\frac{m\_r(t\_{ip})}{N\_r} = \frac{\left(I\_{\text{DLS}} - I\_{\text{DCDLS}}\right)}{I\_{\text{DCMTS}}} = \langle n \rangle \sigma\_n \upsilon\_{4n} t\_{ip} \tag{13}$$

where *n* is the average of injected electrons.

### **3. Experimental procedure**

All GaAsN films were grown by CBE on high conductive *n*- or *p*-type GaAs 2 off toward [010] substrate using Triethylgallium ((C2H5)3Ga, TEGa), Trisdimethylaminoarsenic ([(CH3)2N]3As, TDMAAs), and Monomethylhydrazine (CH3N2H3, MMHy) as Ga, As, and N sources, respectively. The flow rates TEGa = 0.1 sccm and TDMAAs = 1.0 sccm were considered as conventional values. The growth temperatures of 420 C and 460 C were used for *p*-type and *n*-type GaAsN, respectively. Concerning the doping, *p*-type GaAsN films are unintentionally doped. The n-type alloys were obtained using a silane (SiH4) source or by growing the films under lower MMHy and high growth temperature.

Three different device structures are used in this study: (*i*) *n*- and *p*-type GaAsN schottky contacts, (*ii*) *n*+-GaAs/*p*-GaAsN/*p*-GaAs, and (*iii*) *n*-GaAsN/*p*+-GaAs hetro-junctions. The *N* concentration in all GaAsN layers was evaluated using XRD method. Aluminum (*Al*) dots with a diameter of 0.5/1 *mm* were evaporated under vacuum on the surface of each sample. Alloys of *Au-Ge* (88:12 %) and *Au-Zn* (95:05 %) were deposited at the bottom of *n*-type and *p*-type GaAs substrates for each device, respectively. Some samples were treated by postthermal annealing under *N*2 liquid gas and using GaAs cap layers to avoid As evaporation from the surface. The temperature and the time of annealing will be announced depending on the purpose of making annealing. The background doping and the doping profile in the extended depletion region under reverse bias condition were evaluated using the capacitance-voltage (C-V) method. The leakage current in all used samples ranged from 0.3 *n*A to 10 A for a maximum reverse bias voltage of -4 V. A digital DLTS system *Bio-Rad DL8000* was used for DLTS and C-V measurements. The activation energy *Et* and the capture cross section *n,p* were determined from the slope and the intercept values of the Arrhenius plot, respectively.
