Background

Gallium selenide (GaSe) is a native p-type semiconductor that belongs to the III-VI layered semiconductor family like GaS and InSe. Depending on the package type of separate layers and their amount in the unit cell, GaSe crystals can have a structure that corresponds to various polytype modifications. Four polytypes are known for GaSe(β, γ, δ and ε) and their polytypes of unit cell are shown in Fig. 1.1. A great deal of studies have made by several research groups to understand the polytypes in these compounds. The formation of a particular polytype or a mixture of several polytypes depends substantially on the growth method of single crystals. For example, the Czochralski and Bridgman-Stockbarger methods yield mainly the ε polytype. Gas transport reactions also yield the ε polytype with a large number of stacking faults.

Needle crystals of γ, δ and ε polytypes are formed by vacuum sublimation. Single crystals of the β polytype are formed only occasionally. Most of the papers covered in this thesis deal with the ε type. Figure 1.2 shows structure of GaSe, and atomic configuration of GaSe layers. The main structural unit of GaSe is the elementary

layer with two molecules (four atoms) in the unit cell. The atoms are located in the planes normal to the C axis in the sequence Se-Ga-Ga-Se. Each GaSe layer thereby consists of two planes of Ga atoms, which are surrounded on two sides by the unit planes of the Se atoms. The location of atoms inside the layer corresponds to the D_3h¹ group of point symmetry. In this case, three anions form a tetrahedron along with the metal atom. The strong bonding between two sheets of the same layer is covalent with some ionic contribution. But the bonds between the complete four-fold layer is essentially of the Vander Waal type. Due to the characteristics of layer, GaSe exhibits a strongly pronounced structural anisotropy. Consequently, the ε type GaSe is a promising candidate material for nonlinear optical conversion devices in the near- to far-infrared wavelength(1-18µm), and the intermediate layer to connect such lattice-mismatched semiconductors as GaAs and Si with reduced number of misfit dislocations by the van der Waals epitaxy. Besides the removal of the constraint of the lattice mismatch, GaSe thin film possesses the advantages of stability against heating and oxidation under the ultra-high vaccum condition. GaSe has recently been reported to be applicable as a termination layer as well as an electronic passivation surface on Si even limited to nano-scaled structures.

Due to its relatively large band gap energy of 2.0 eV, impurity doping in GaSe has been investigated with much interest because of its possible applications for photo-electric devices in the visible region. The photo-electric and optical properties of GaSe doped with elements of groups I, II, IV, and VII have been reported by many researchers.

The hole concentration on the order of 10¹⁵− 10¹⁶ cm⁻³ at room-temperature have been demonstrated by doping Cd[1], Zn[2], Cu[3], Mn[4], and Ag[5]. Activation en-ergies for hole concentration are of the order of 300 meV for Cd, Zn, and Mn doped

Figure 1.1: Polytypes of GaSe

GaSe, of the order 40 and 140 meV for Cu-doped samples, and of the order of 60 meV for Ag-doped samples. Until now only the dopants of Sn[6], Cl[7, 8] and I could act as donors in GaSe samples. The electron concentration for n-type GaSe are the order of 10¹²− 10¹³ cm⁻³ at room-temperature. Attempts to get low resistivity n-type GaSe have been unsuccessful.

Except the above elements, much attention has been recently paid to GaSe doped with transition-metal elements and its optical properties. Trivalent rare-earth ions are well known for their special optical properties, which result from the factor that the electrons of the partially filled 4f shells are shielded from the surrounding completely filled 5s and 5p shells. The energy levels of the 4f shell have equal parity, and hence

Figure 1.2: Layer structure of GaSe

electric dipole transitions are forbidden. In a solid, the slight mixing with odd-parity wavefunctions makes transition slightly allowed. The influence of the electric field around the ion removes the degeneracy of the 4f -levels, resulting in a Stark-splitting of the energy levels. However, due to the shielding by the outer lying shells, the magnitude of the splitting is small, resulting in relative narrow emission lines, of which the wavelength is almost independent of the host material. The energy levels of the 4f shells arise from spin-spin and spin-orbit interactions and are often denoted using Russel-Saunders notation ^2s+1L_J, in which S is the total spin angular momentum, L is the total orbital angular momentum quantum number and J is the magnitude of the total angular momentum, J=L+S according to vector model. There exist 14 rare-earth elements, that all have a different number of electrons in the incompletely filled 4f shells. As a result, each rare-earth ion has its own specific energy levels, and hence typical luminescence lines. The rare-earth ion erbium has transition at

1.54 µm, which is the standard wavelength used in optical telecommunication. The effect of doping with Gd[9], Yb[10], Dy[11], and Tm[12] on optical behaviors of GaSe crystals was studied.

GaSe possesses a number of exciting properties, which are listed in Table 1.1, for nonlinear optical application. Among these nonlinear optical crystals, GaSe has a transparency range extending from a wavelength of 0.65 to 18 µm where the optical absorption coefficient dose not exceed 1 cm⁻¹ throughout the range. The ε type GaSe is a negative uniaxial crystal (n_o > n_e, where n_o and n_e denote the refractive indices in the ordinary and extraordinary direction). Its nonlinear optical coefficients are among the top five for birefringent crystal. Due to its large birefringence, it can satisfy phase matching (PM) conditions for optical configurations within the nonlinear optical crystals. Recently, incoherent parametric generation tunable in the range of 3.5-18 µm in GaSe (type-I PM) was obtained by using 110 ps pulses from actively mode-locked Er:YAG laser as a pump source[13]. Subsequently, picosecond pulses of mode-locked Er:Cr:YSGG laser were used to pump a traveling-wave optical parametric generation(OPG); type-I and type-II OPG provided continuous tunability in the range of 3.5-14 and 3.9-10 µm, respectively[14]. On the other hand, there have been a number of reports on difference frequency generation (DFG) to achieve tunable and coherent mid-IR for GaSe by using variety of laser sources.[15, 16] Additionally, few papers reported on THz-wave generation from GaSe.[17, 18, 19] Because GaSe has lowest absorption coefficients in the THz wavelength region. Consequently, GaSe has the largest figure of merit for the THz generation (d²_eff/n³α²), which is several orders of mabnitude larger than that for bulk LiNbO₃ at 300 µm. According to Y.J.

Ding’s results, an efficient and coherent THz wave tunable in the two extremely wide

Nonlinear Merit Transparency Absorption Damage Coefficient Factor Range Coefficient Threshold Crystal (pm/V) d²/n³ (µm) (cm⁻¹) (M W/cm²)

ZnGeP₂ 68.9 162 0.74-12 0.83 60

AgGaSe₂ 32.6 63.3 0.71-18 0.089 2

AgGaS₂ 12.5 12.8 0.47-13 0.04 25

GaSe 54 217 0.65-18 0.081 35

T l₃AsSe₃ 36.5 42.4 1.28-17 0.082 16

CdGeAs₂ 217 1090 2.4-18 0.5 40

Table 1.1: Nonlinear optical crystals for mid-IR applications

ranges of 2.7-38.4 and 58.2-3540 µm, with typical linewidths of 6000 MHz, has been achieved for the first time.