Fabrication and Electrical Characterization of AlGaN/GaN HFETs

Transcription

1 Fabrication and Electrical haracterization of lga/ga HFET 1 Michael. waah, Iibhakhomen waah, Okechukwu kpa, 3 Uwadiae Obahiagbon and 4 itu. palangya 1 Faculty Member, HSS Grand anyon Univerity, 33 W. amelback Road Phoeni Z 8517 US Senior Proce Engineer, 33 E Yellowtone Place, handler rizona 8549 US 3 Graduate Student, Electrical Engineering ept., rizona State Univerity. Tempe, Z, 8581 US 4 Lecturer, ept. of Food Proce Engineering, School of Engineering, Univerity of Ghana, Legon, Ghana Received: / ccepted: btract: i/lga/ga rectifying diode tet tructure wa fabricated, repreentative of the gate element of heterotructure FET HFET with annealed ohmic back contact and HFET device of variou geometrie. Electrical meaurement on TLM pad tet produced a contact reitivity of. 1-3.cm, a barrier height of ~1.1 e for the rectifying diode, a heet charge denity of ~ cm - at the lga/ga interface, and a conduction band offet of.7 e. Thee value are in cloe agreement with thoe reported by other reearcher. The HEMT device yielded a drift molity of ~511 cm / and a tranconductance of 1 5 ms. Keyword: Ga HEMT, Ga HFET lga HEMT, lga HFET, HEMT device, Tranconductance, rift Molity, ontact I. ITROUTIO Gallium nitride Ga poee everal outtanding phyical propertie, which make it particularly attractive for olid-tate device application. In particular, the Ga baed material ytem i of great interet for blue/green light-emitting diode LE, laer diode, U photodetector, and alo for the realization of high freuency and high-power electronic device for operation at elevated temperature. a wide direct bandgap material E g 3.4 e, Ga ha a very high breakdown field approimately 3 M/cm [1, ], high aturation velocity, 1 7 cm/, and high heet charge concentration in the lga/ga interface. The high breakdown field allow Ga-baed device to be aed at very high drain voltage break-down = 5 5 depending on the application of the device. The high aturation electron velocity contribute to a higher current denity ince, I ma n, where i the electronic charge, n i the heet charge denity, i the electron aturation velocity, and alo a high operating freuency, ince f / L. t eff The eitence of the lga/ga heterotructure lead to a high heet charge denity n = /cm, which contribute to a high I ma ; wherea the high electron molity 1 15 cm /. contribute to a low on-reitance, ince channel reitance i related to 1 / n E at low electric field. Thee phyical propertie of Ga tranlate into fundamental performance advance in the area of high power, high freuency power tranitor for RF application covering over a 75 GHz band. Group-III nitride ehit fairly high bond trength and very high melting temperature. The large bond trength could poibly inhit dilocation motion and improve reliality in comparion to other II-I and III- material [3]. In addition, the nitride are reitant to chemical etching and hould allow Ga-baed device to be operated under harh environment. vailable 11

2 n ideal heterojunction conit of a emiconductor crytal in which there eit a plane acro which the identity of the atom contituting the crytal change abruptly. In reality, the ideal tructure i approached uite cloely in ome ytem uch a molecular beam epitay MBE grown l Ga 1- /Ga heterojunction with eentially atomically abrupt interface [3, 4]. In the reported work, iotype heterojunction between unintentionally doped n-type lga and Ga are conidered. When two n-type emiconductor with different bandgap and work function are brought into contact, in order to etablih an euilibrium, electron flow from the wider to the narrower bandgap material reulting in an alignment of the Fermi level with aociated band bending and band dicontinuity, a indicated in Figure 1. Thee electron accumulate in a narrow region of lowet energy a uantum well at the interface between the two emiconductor forming a heet of charge, generally referred to a two-dimenional electron ga EG. Electron in the EG ehit much higher molity than the bulk material forming the heterotructure a they are phyically eparated from the donor in the material cauing minimal impurity cattering. EG Figure 1. Band dicontinuity in a unipolar heterotructure The heet charge denity at the lga/ga interface i further enhanced due to pontaneou polarization ariing from the aymmetry eiting in the lga and Ga crytal tructure and alo by piezoelectric polarization due to train at the heterointerface [4]. The total charge at the lga/ga interface can eceed cm -, which i roughly five time higher than the lga/ga heterotructure. Thi high denity EG in the lga/ga ytem ha been uccefully employed a the conducting channel in the fabrication of a cla of high performance field effect tranitor, commonly referred to a high electron molity tranitor HEMT [5-9]. II. METHOOLOGY Tet tructure were fabricated uing two et of photo mak. The firt, a et of three mak for the fabrication of Ohmic and rectifying contact, the econd et, a et of five mak deigned a a part of the preent tudy, wa ued for the fabrication of HFET device. The fabricated device on the heterotructure were electrically characterized. For the tudy of ohmic contact, rectangular pad meauring 1 m 4 m with uneual pacing 3 m, 5 m, 1 m, 15 m, m, 5 m, 3 m, 35 m, 4 m, 45 m between contact were ued, a hown in Figure. Thi enabled the meaurement of the reitivity uing the tranfer length method TLM. ontact reitivity wa obtained from plot of meaured reitance between two adjacent pad a a function of pad eparation. vailable 1

3 Z L Figure. chematic diagram of TLM contact pad ued for the meaurement of the contact reitance Schottky contact ued for the tudy of rectifying characteritic conit of two et of circular contact of varying diameter et 1: 1,, 3 and 4 and et: 15, 5 and 35 m urrounded by 15 and 5 m wide ring-haped ohmic contact with prong pad. The firt mak define the ring-haped ohmic contact. The econd mak define circular Schottky contact within the area bounded by the ring-haped ohmic contact. The third mak defined the contact pad. dditional thickne of u wa depoited on the contact pad. The ample ued for thi reearch were MBE grown material obtained from ST ociate, Eden Prairie, Minneota. The heteroepitaial film of l. Ga.8 /Ga on apphire conited of nm unintentionally doped l. Ga.8 grown on 1 m unintentionally doped Ga, with donor concentration 15 of 11 cm -3 [1]. grown Ga film how n-type characteritic. ative point defect and nitrogen vacancie are thought to be primarily reponible for the n-type characteritic of the a grown film. The " diameter wafer were diced into 5 mm 5 mm ample by the vendor. The a-received ample were cleaned in acetone and methanol and rined in deionized I water. Thi removed organic contaminant from the ample. Sample were then treated in a buffered oide etchant BOE olution for 5 min. to remove oide film that might have been preent on the urface of the ample. multi-layer Ti/l/i/u 15/6/35/5 nm metallization wa choen for ohmic contact. Thi multi-layer metallization wa choen in order to obtain low pecific contact reitivity, good urface adheion and long life-time [11]. Schottky contact wa defined by lift-off of i/u /18 nm. evice fabrication wa completed with a depoition of a paivation layer. The Ti layer wa depoited to help in the formation of n + - Ga interface and provide a good adherence to the urface. u layer wa for preventing the oidation of the l and a better conductivity of the metal layer, and i layer wa utilized for reducing of the u/l intermiing. III. RESULTS ISUSSIO From reitance meaurement uing TLM pad, contact reitivity of ohmic contact and heet reitivity of the lga/ga film were determined. The barrier height of the rectifying i/lga/ga contact wa determined from I- and - meaurement. apacitance-voltage meaurement were alo ued to determine conduction band dicontinuity, polarization charge and heet charge denity and the carrier concentration profile at the lga/ga interface. evice parameter uch a tranconductance and drift molity were etracted from the meaurement on the lga/ga fat FET device. Multi-layer Tiliu contact in the a-depoited tate were emi-ohmic. n anneal treatment at a temperature of 85 o for 3 ec. in ament wa performed in order to form the ohmic contact. The Ti anneal form Ti interfacial layer reulting in vacancie in the Ga thereby cauing an increae in the donor concentration. vailable 13

4 Sheet reitance of about /, contact reitance of about and contact reitivity value of cm were etracted from the plot of total reitance a a function of the pad eparation. urrent-voltage characteritic of the i/lga/ga rectifying contact were plotted and analyzed. For the contact diode where thermionic emiion i the dominant current conduction mechanim, the current denity can be epreed a [1]: J J S [ep / 1], or J J ep / n for 3 /, 1 where * b J T ep, and n i the ideality factor given by n, ln J 3 T i the temperature, the applied voltage, and * the effective Richardon contant. The theoretical value of the effective Richardon contant i given by [13]: * * m k, 3 h 4 where h i the Planck contant and m *, the effective ma. For a given, the effective ma i etimated by linear interpolation from the theoretical value m * =.35m o for l and m * =.m o for Ga [14, 15]. linear plot of the I- characteritic obtained from i/lga/ga Schottky contact for variou diameter howed a turn-on voltage between.3 and.7. The 4 m device indicated a revere leakage current of /cm at., repectively. Semilogarithmic plot of the forward characteritic of the contact, a hown in Figure 6 ehited a linear region with an ideality factor ~1., indicating that current conduction i dominated by thermionic emiion. alculated ideality factor and barrier height uing euation 3 and 4 are value are ummarized in Table 1. Table 1. Ideality factor and barrier height Schottky ontact, Revere current Ideality factor, n Barrier Height, b, e m I R, at, /cm vailable 14

5 1 - urrent um 3 um um 1 um o lta ge Figure 3. Semilogarithmic plot of the forward characteritic of i/lga/ga Schottky diode apacitance voltage characteritic of the Schottky contact apacitance-voltage meaurement were obtained for the i/lga/ga rectifying diode uing an HP484 LR meter with 5 m peak-to-peak voltage at a freuency of 1 MHz. Reult obtained are hown in Figure 4. Thee - plot were ued for the determination of conduction band dicontinuity, polarization charge denity, heet carrier concentration and apparent carrier ditribution profile at the lga/ga interface. 1 4 um 3 um 3 um 1 um log lo g Figure 4. apacitance - voltage plot for i/lga/ga rectifying contact The capacitance of a Schottky contact i given by [13]: B 1/ 5 vailable 15

6 vailable / B, 6 where i the built in potential,, the area of the Schottky contact, B, the background doping concentration. Euation 6 can be rewritten a B, or B /. 7 plot of the invere capacitance uare 1/ veru revere a voltage i hown in Figure 8. The traight line in Figure 5 i given by: / 1 B. 8 The lope of the plot of / v. i given by: B lope. 9 From the graph in Figure 6, the built-in potential, depletion width, apparent carrier concentration and heet carrier concentration can be determined. The apparent carrier ditribution n * i given by [16] lope n B. 1 The depletion width, w i given by: p n w, 11 where, 1/ p, 1 1/ n. 13

7 3 y = , m a dev:.937 1/ 1/pF oltage, For n-n unipolar heterotructure it wa aumed that and lga Ga 1/ w. 14 urrent-voltage data of circular gate, -gate, fatfet HEMT and Schottky diode have been collected with an HP4155 and plotted. With gate voltage, G from to 4 at a tep of.4 and gate-to-drain voltage of up to 5, a drain current greater than 4.3 m wa obtained. For an n-channel mode tranitor, a negative potential on the gate i reuired to turn the tranitor channel off. plot of the drain current a a function of drain voltage i hown in Figure 6. The I- curve how a linear region where the drain current varie linearly with the applied grain voltage. Thi region i alo known a the ohmic region with d << d,at. The HFET device have typical I- characteritic for Ga a they ehit ome high field effect at the larger gate voltage. From the plot in Figure 1, tranconductance and drift molity value were etimated. t both high and low freuencie, the output drain current of a tranitor i characterized by a current generator g m g, in parallel with the output conductance g d [14, 15]. The tranconductance g m in a device i given by: g I i d Figure 5. 1/ v. plot for 4 m rectifying contact d m 15 GS GS w = a d n d 16 l b where d i the drift molity at a depth and n i the carrier concentration, w the width and l the length of the channel. vailable 17

8 g= g= -.4 urrent m g= -.8 g= oltage g= -1.6 g= -. g= -.8 g= -.4 Figure 6. I v. characteritic of lga/ga HFET 1 µm Tranconductance can alo be given by: w R c g m d n dd 17 l RT v gg where R i the channel reitance and R T i the total reitance. From above relationhip, tranconductance of ~1 5 ms were obtained for the curve that ehited a low field effect. The threhold voltage of -.97 wa etracted. The average carrier concentration over a thickne of cm wa determined to be n cm -3. drift molity of approimately 511 cm / wa etimated. I. OLUSIO Field effect tranitor were fabricated on MBE grown film of l. Ga.8 /Ga heterotructure. pecific contact reitance of cm and a heet reitance of /cm wa obtained for putter depoited TLM pad which conited i/u /15 Å for p-lga and Ti/u /15 Å lga/ga. The contact reitance wa high, but that wa epected a the ubtrate wa undoped. However, the heet reitance wa three order of magnitude lower than the reult reporter by waah for the ame material ytem [1]. Rectifying contact were obtained by depoiting i 15 Å on the n-ga. Ideality factor etracted from the current voltage meaurement were ~1., indicating that thermionic emiion wa the dominant tranport mechanim. barrier height of ~.9 e wa determined for the i/n-ga rectifying contact from I- meaurement. nalyi of - characteritic yielded an apparent carrier concentration of cm -3 and a heet carrier concentration of cm -. Thee value are in good agreement with thoe reported by other reearcher and the manufacturer of the ubtrate. Fabricated lga/ga HFET device ehited tranitor behavior with tranconductance value ranging from 1. to 9.3 ms/mm. The threhold voltage wa -.97 and a channel molity of 484 cm / wa obtained from the calculated tranconductance. The n-ga MESFET fabricated on vailable 18

9 anneal treated ubtrate yielded tranconductance value between.5 and.13 ms/mm. Thee device achieved low aturation current but no cut-off. REFEREES [1] S. akamura and S.F. hichibu,. Introduction to itride Semiconductor Blue laer and Light Emitting iode, R pre, Boca Raton. [] K. Linthicum, Ga Overview, T.W. Staley and R.J. Matyi, " Statitical Method for the Fitting of ouble rytal X-ray Rocking urve", J. ppl. rytallography, vol. 3, p. 368 [3] J.. Brown, R. Borge, E. Piner,. ecan, S. Singhal, R. Therrien,. lga/ga HFET fabricated on 1-mm Ga on ilicon 111 Subtrate, Solid State Electronic, vol. 46, pp [4] U. K. Mihra, P. Parikh, and Y. Feng Wu,. lga/ga HEMT- n Overview of evice Operation and pplication, Proceeding of the IEEE, vol. 9, no. 6, p. 1. [5] W. Lu,. Kumar, E. Piner, and I. deida, 3., RF, and microwave noie performance of lga/ga field effect tranitor dependence of aluminum concentration, IEEE Tran. Electron evice, vol. 5, pp [6] J.. Smart,. T. Schremer,. G. Weimann, O. mbacher, L. F. Eatman, and J. R. Shealy, Growth of piezoelectric HEMT tructure on apphire and ilicon carde applying inulating lga nucleation layer, ppl. Phy. Lett., vol 75, p [7] M.. Khan, Q. hen, M. S. Shur, B. T. Mcermott, J.. Higgin, J. Burm, W. J. Schaff, and L. F. Eatman, "W Operation of Short-hannel Ga/lGa oped hannel Heterotructure Field Effect Tranitor at 1 GHz and 15 GHz", IEEE Electron evice Lett., vol. 17, no. 1, pp [8] R. Gaka, Q. hen, J. Yang,. Oinky, M.. Khan, and M. S. Shur, High-Temperature Performance of lga/ga HFET' on Si Subtrate", IEEE Electron evice Lett., vol. 18, no. 1, pp [1]. aram, 5. ST ociate private communication. [11] H.-Ki Kim, T-Y. Seong, I. deida,. W. Tang, K. M. Lau. 4. Low-reitance Pt/Pd/u ohmic contact to p-type lga, ppl. Phy. Lett., vol. 84, p. 1. [1]. M. rowley and S. M. Sze Surface State and Barrier Height of Metal- Semiconductor Sytem. J. ppl. Phy., vol. 36, p. 31 Intercience, Y. [13] S. M. Sze, 1981.Phyic of Semiconductor evice, John Wiley & Son. [14] B. L. nderon, and R. L. nderon, 5. Fundamental of Semiconductor evice, McGraw Hill. [15] R.. Pucel, and. F. Krumm, Simple method of meauring drift-molity profile in thin emiconductor film, Electronic Lett., vol. 1, no. 1, p. 4. vailable 19