MEAS URED RE SULTS FOR NbN PHONON- COOLED HOT ELEC TRONBO LO MET RIC MIX ERS AT 0.6-0.75 THz, 1.56 THz, AND 2.5 THz

E. Gerecht, C. F. Mu sante, H. Jian, and K. S. Yngves son

De part ment of Elec tri cal and Com puter En gi neer ing, Uni ver sity of Mas sa chu setts,Am herst, MA 01003

J. Dick in son and J. Wald man

Sub mil li me ter Tech nol ogy Labo ra tory, Uni ver sity of Mas sa chu setts at Low ell Re search Foun da tion,Low ell, MA 01854

G.N. Gol’ts man, P.A. Ya goubov, B.M. Vo ro nov, and E.M. Gershen zon

Department of Physics, Mos cow State Peda gogi cal Uni ver sity,Mos cow, Rus sia

I. IN TRO DUC TIONNbN Hot Elec tron Bo lo met ric (HEB) mix ers rep re sent a prom is ing ap proach for achiev ing re -

ceiver noise tem pera tures of a few times the quan tum noise limit at fre quen cies above 1 THz. These HEB

mix ers have so far dem on strated a DSB noise tem pera ture as low as 500 K at 630 GHz [Kroug et al., 1997;

Ka wa mura et al., 1997], and 980 K at 900 GHz [Kroug et al., 1997]. Noise tem pera tures of about 1000 K or

less can be ex pected for fre quen cies above 1 THz in the fu ture. NbN HEB mix ers have been shown to have

suf fi cient band widths for the an tici pated ap pli ca tions such as fu ture re ceiver fron tends for THz as tro nomi -

cal ob ser va tion from space. A re ceiver noise band width of 5 GHz and con ver sion gain band width of 3 GHz

were meas ured by [Ek ström et al., 1997]. The LO power re quired is typi cally 100 nW, which makes the

NbN HEB mix ers suit able for use with fu ture solid state tun able THz sources. How ever, the LO power is

not at such a low level that its op er at ing point is af fected by in put ther mal noise power. This pa per de scribes

the de vel op ment of THz HEB mix ers for the 1 THz to 2.5 THz fre quency range. The HEBs em ploy NbN as

the bo lo met ric ma te rial. We pres ent our first re sults from meas ure ments with these mix ers at fre quen cies of

0.6 THz to 0.75 THz, 1.56 THz, and 2.52 THz. We have meas ured a DSB re ceiver noise tem pera ture of

5,800 +−14001000 K at 1.56 THz. In a sepa rate ex peri ment, we meas ured a con ver sion gain of 3 dB ±2 dB at the

same fre quency.

II. DE VICE DE SIGN AND FAB RI CA TION

NbN FilmsThe NbN films were fab ri cated on sili con sub strates at Mos cow State Peda gogi cal Uni ver sity

(MSPU) by mag ne tron re ac tive sput ter ing in an ar gon/ni tro gen gas mix ture. For this work we have pri mar -

ily used films of thick ness 3.5-4 nm in or der to maxi mize the con ver sion gain band width. The pro duc tion of

such thin films is still an evolv ing tech nol ogy, but re cent films on both sap phire and sili con sub strates have

shown much im proved prop er ties [Chered nichenko et al., 1997]. The op ti mum thick ness, based on the sap -

phire work, ap pears to be close to 3.5-4 nm. The films used in thease measurements have Tc =7.5 - 9 K, the

tran si tion width is about 1 K, and the criti cal cur rent den sity is 2 x 106 A/cm2.

Op ti cal De signOp ti cal de sign con sid era tions are cru cial for ef fi ciently cou pling LO and sig nal power into the de -

vice. Quasi- optical cou pling to the de vice is most com mon for fre quen cies above 1 THz. We chose to use an

ex tended hemi spheri cal sili con lens cou pled to a self- complementary log- periodic an tenna (see Fig ure 1),

as suc cess fully dem on strated and ana lyzed at 250 GHz and 500 GHz by [Filipo vic et al, 1993]. The log-

periodic an tenna is con ven ient since it can be used over a very wide fre quency range; later ver sions will em -

ploy twin- slot or twin- dipole an ten nas tuned to smaller fre quency bands. We scaled the di men sions of the

lens and the an tenna used in the 250 GHz setup by a fac tor of ten, re sult ing in a lens di ame ter of 1.3 mm. We

chose an ex ten sion length, be yond the hemi spheri cal lens, of 0.33 times the lens ra dius. We can pre dict the

amount of beam- scan which would be caused by mis align ment of the cen ter of the an tenna with re spect to

the cen ter of the lens: a 20 mi crome ter mis align ment re sults in a 5 de gree beam scan. This makes it im pera -

tive to use an ac cu rate align ment pro ce dure, which will be de scribed be low. We are not em ploy ing a match -

ing layer for the lens at this stage.

De vice Fab ri ca tionDe vices have been fab ri cated at MSPU as well as at UMASS/Am herst. The pro cesses are some -

what dif fer ent at the two lo ca tions, but in what fol lows we will em pha size the UMASS pro cess. The gold

log- periodic an tenna is fab ri cated us ing lift off. Af ter the pat tern has been de fined in the pho to re sist, a 40 nm

thick layer of Nb is ap plied by sput ter ing. Next, 20 nm of Ti and 100 nm of Au are de pos ited by E- beam

evapo ra tion, and the lift- off is per formed. The NbN strips are then de fined and etched us ing Re ac tive Ion

Fig ure 1: Log- periodic an tenna fab ri cated on an ex tended hemi spheri cal lens.

Etch ing (RIE). The sub strate is thinned by lap ping to a thick ness equal to the lens ex ten sion length. The po -

si tion of a square align ment win dow for the lens is de fined in a pho to re sist layer on the op po site side of the

sub strate from the an tenna and de vice, us ing an in fra red mask aligner, where upon the align ment win dow is

etched by RIE to a depth of 100 nm. The lens is at tached to the sili con sub strate us ing pu ri fied bees wax. The

fi nal di men sions of the de vice strips are about 0.6 µ long by 1.0 µ wide. The number of strips is from one to

three. The mask also has a dif fer ent pat tern for which all teeth are larger by a fac tor of two, i.e. the small est

teeth de ter mine a high est fre quency of the an tenna of about 1.25 THz. This an tenna can have up to five

strips.

III. EX PERI MEN TAL SETUP

Re ceiver Con figu ra tionThe in te grated an tenna/HEB de vice and lens are at tached to a cop per post, which is ther mally an -

chored at the other end to the liq uid he lium res er voir of an IR LAB de war (see Fig ure 2 and Fig ure 3). The

an tenna is con nected to the IF and bias sys tem via a mi crostrip/semiri gid coax line and bias tee. A cooled

HEMT am pli fier with iso la tor in put is also used in side the de war (Fig ure 3). This IF am pli fier has a band -

width from 1250 to 1750 GHz, with noise tem pera ture of about 10K.

Op ti cal SetupThe op ti cal cou pling loss as well as the re ceiver noise tem pera ture are meas ured with a CO2 la ser

pumped FIR metha nol la ser as the LO source. The la ser setup is il lus trated in Fig ure 4. My lar beam split -

Fig ure 2: Pho to graphs of the OFHC cop per ped es tal: (a) close- up of the de vice mount and con tacts, (b) close- up of the op po site side of the ped es tal show ing the sili con lens.

(a) (b)

ters with a thick ness of 6, 12.5, and 25 mi crome ters, re spec tively, act as di plex ers be tween the LO and a

chopped hot/cold noise source. The ra dia tion is fo cussed by an off set pa rabo loi dal re flec tor or a TPX lens.

In or der to meas ure the con ver sion gain di rectly, we em ploy two la sers at UMASS/Low ell as shown in Fig -

ure 5. The ac tive me dium of these la sers is difluoro meth ane, and the fre quency 1.56 THz. The la sers were

slightly de tuned and pro duced an IF of 600 kHz. The gain band width of NbN HEB de vices at the IF can not

be eas ily meas ured at THz fre quen cies. At pres ent, we meas ure the mixer gain band width at 94 GHz in -

stead. Since the LO power re quired is so low, the same op ti cal con figu ra tion (in clud ing the lens), can be

used at 94 GHz also, al though the an tenna re sponse does not ex tend this low in fre quency.

Noise tem pera ture meas ure ments at 0.6 THz to 0.75 THz were also per formed at Chalmers Uni -

ver sity in a setup which used a BWO as LO source and a 12.5 µm beam split ter . The ex peri men tal ar range -

ment was as de scribed in [Ekström et al., 1997].

Fig ure 3: Top view of the IR LAB de war show ing the dc and IF con nec tions.

FTS Spec tra of the De vice Re sponseFou rier Trans form Spec tra were ob tained at Chalmers Uni ver sity by em ploy ing the de vice as a de -

tec tor, at a tem pera ture close to Tc.

IV. RE SULTS AND DIS CUS SION

FTS Spec tra of the De vice Re sponseFig ure 6 shows the spec tra ob tained for a NbN de vice in te grated with the larger ver sion of the log-

periodic an tenna de scribed above. The de vice was mounted in two per pen dicu lar ori en ta tions in or der to

elu ci date the frequency- dependence of its op ti mum po lari za tion. [Kor man yos et al., 1993] showed that the

po lari za tion for op ti mum re sponse var ies pe ri odi cally with fre quency at an am pli tude of ±22.5°. This is

con sis tent with the spec tra we ob served, in which dips in the re sponse oc cur at fre quen cies which are one

oc tave apart. As the ori en ta tion of the de vice was changed by 90 de grees, peaks ap pear where dips oc cur for

the per pen dicu lar ori en ta tion. The high est fre quency peak cor re sponds to when the sec ond small est tooth is

one quar ter wave length long when con sid er ing the ef fec tive di elec tric per mit tiv ity of the sili con me dium. In

or der to util ize self- complementary log- periodic an ten nas, one has to be aware of their sen si tiv ity to the in -

ci dent po lari za tion. The power avail able, when la sers are used as the LO sources, is in general suf fi ciently

large such that a po lari za tion ro tater can be used to pro duce the op ti mum po lari za tion. This was con firmed

in our ex peri ments at both 1.56 THz and at 2.52 THz.

Fig ure 4: Op ti cal setup for meas ure ment of op ti cal cou pling loss and re ceiver noise tem pera ture.

Noise Tem pera ture Meas ure mentsOur re sults from the noise tem pera ture and con ver sion loss meas ure ments at dif fer ent fre quen cies

are sum ma rized in Ta ble I be low.

Ta ble I: Noise Tem pera ture and Con ver sion Loss Sum mary

f (GHz) TDSB

(K) Tout

(K) TIF

(K) Tmix

(K) Lc,tot

(dB) Lc,i

(dB) Lc,opt

(dB)

94 NA NA NA NA NA 9 NA

600-750 900 105 24 730 12.7 7.7 5

1,560 5,800 10 13 2,500 27 18-20 7-9

The Y- factor was meas ured by in sert ing a liq uid ni tro gen cooled ab sorber by hand into the path of

the beam sev eral times. In the 1.56 THz ex peri ment, the IF out put power was re corded on a chart re corder,

and the re sults of many in di vid ual Y- factor meas ure ments were av er aged. The Y- factor was de ter mined to

be 0.155 dB ±0.03 dB, which yields a DSB re ceiver noise tem pera ture of 5,800 +−14001000 K. The fact that it was

pos si ble to per form the Y- factor meas ure ment at this noise tem pera ture level with out the use of a ro tat ing

chop per is a trib ute to the ex cel lent am pli tude sta bil ity of the UMass/Low ell la ser used for this ex peri ment.

The sta bil ity is also evi dent in the I-V curves re corded by our fast (about 1 ms) com put er ized re cord ing sys -

tem. Fig ure 7 shows the op ti mum op er at ing point of the de vice in the noise tem pera ture measurement. Near

Fig ure 5: Op ti cal setup for meas ure ment of con ver sion gain.

the op ti mum op er at ing point ( 0.97 mV, 38 µA), the de vice is very sen si tive to varia tions in LO power.

Nev er the less, only very small varia tions are evi dent in this re cord ing. In con trast, the I-V curve ob tained

with the free- running 2.52 THz la ser shows large fluc tua tions in the sen si tive re gion of the I-V curve (see

Fig ure 8). The an tenna/lens com bi na tion clearly per formed well at 1.56 THz, as evi denced by the fact that

dur ing a chopped noise meas ure ment on the sys tem, it was pos si ble to blank out es sen tially the en tire sig nal

by block ing the cold source with a piece of ab sorber of a size equal to the pre dicted beam width, about 8 de -

grees at this fre quency. The LO power ab sorbed by the de vice was 240 nW (for the 600- 750 GHz de vice it

was about 100 nW). There was no meas ur able change in the DC op er at ing point when the in put load of the

mixer was changed from room tem pera ture to liq uid ni tro gen tem pera ture.

Ta ble I shows the es ti mated break- down of the to tal con ver sion loss into com po nents. We as sume

that the op ti cal cou pling loss is es sen tially given by the known losses of dif fer ent com po nents, such as the

polyeth yl ene win dow, the Zitex ther mal ra dia tion fil ter, the re flec tion loss of the lens, etc.. The re main ing

loss to be ac counted for by the mixer it self, the in trin sic con ver sion loss, in clud ing IF out put mis match, is

about 18- 20 dB for the 1.56 THz mixer. This some what high in trin sic loss can be ex plained by the HEB the -

ory [Ekström et al., 1995] and the spe cific I-V curve for this de vice when ir ra di ated by the 1.56 THz LO,

tak ing into acount the IF out put mis match. Nev er the less, a de tailed ex pla na tion will have to await the com -

ple tion of a new, more com plete, theo reti cal model for the HEB de vice [Mer kel et al., 1998]. It is in ter est -

ing to com pare the I-V curves re corded with LO power at dif fer ent fre quen cies in Fig ure 8, how ever. Ra -

dia tion at both 1.56 THz and 2.52 THz should be above the band gap fre quency of the NbN film, and thus it

Fig ure 6: FTS spec tra of the de vice for (a) de vice ori en ta tion par al lel to the FTS po lari za tion, and (b) per -pen dicu lar to the FTS po lari za tion.

would be ex pected that both fre quen cies should be uni formly ab sorbed by the de vice. Yet, the I-V curves

dif fer no ticea bly when the de vice is ir ra di ated by 1.56 THz and 2.52 THz. The curves with 356 GHz and 94

GHz ra dia tion are quite dif fer ent from the higher fre quency curves and also dif fer among them selves. Note

that all I-V curves were re corded for the same de vice with out chang ing the de vice con figu ra tion. Nei ther of

the I-V curves in Fig ure 8 agrees with curves re corded with out LO power at higher de vice tem pera tures,

which gives an other in di ca tion that the HEB model needs to be re fined. HEB mixer the ory pre dicts a 10 dB

in trin sic con ver sion loss with an LO at 94 GHz which is in good agree ment with the meas ured value of 9 dB

(Ta ble I). The ex act shape of the I-V curve thus is very im por tant for the ac tual per form ance of the de vice in

terms of con ver sion loss. Note that dif fer ent de vices (phonon- cooled or diffusion- cooled, op er ated at dif -

fer ent fre quen cies) of ten dif fer dras ti cally in terms of the in trin sic con ver sion loss as well as the out put noise

level from the de vices. Both con ver sion loss and out put noise are rele vant in es tab lish ing the re ceiver noise

tem pera ture and a com pari son of the ul ti mate po ten tial re ceiver noise per form ance of dif fer ent types of de -

vices will re quire the more de tailed un der stand ing of the de vice mod els we re fer to above. It should be

clear, how ever, that re ceiver noise tem pera tures of HEB mix ers at THz fre quen cies are likely to pro gres -

sively get lower. Our own meas ure ment of 5,800 K at 1.56 THz was for a sin gle de vice, the only one tested

so far. Ex pe ri ence at the lower fre quen cies has al ready shown that re fine ment in the re ceiver con figu ra tion,

as well as in de vice fab ri ca tion, will gradu ally lead to lower noise tem pera tures.

Meas ure ment of Con ver sion GainThe in trin sic (de vice only) con ver sion gain at 1.56 THz was meas ured in the two- laser setup to be

+3 dB, with a prob able er ror of ±2 dB. The de vice used for this meas ure ment was fab ri cated in Mos cow

Fig ure 7: I-V curve for the de vice used in the 1.56 THz noise tem pera ture meas ure ment.

and util ized an equi an gu lar spi ral an tenna. The ab sorbed power from the RF la ser was ob tained by the tech -

nique we em ployed for the ab sorbed LO power at a high enough RF power level to make this pos si ble. Cali -

brated attenua tors were then used to lower the RF power un til the mixer was shown to be op er at ing in its lin -

ear re gion. The IF power was ob served on a spec trum ana lyzer and the IF volt age was meas ured di rectly on

an os cil lo scope. The op ti cal cou pling loss was es ti mated to be about 33 dB in this case. Note that HEB mixer

the ory al lows ac tual con ver sion gain to be re al ized. The con ver sion gain at higher IF fre quen cies may be

some what lower; so far, the best in trin sic con ver sion gain of any HEB THz mixer at about 1 GHz IF, in -

ferred from noise meas ure ments, is about – 6 dB (Kroug et al., 1997). The ap par ent dif fer ence in con ver -

sion loss at dif fer ent IF fre quen cies in di cates an other area in which our pres ent de vice mod els are in ade -

quate.

V. CON CLU SIONWe have dem on strated re ceiver noise tem pera tures of 900 K at 600- 750 GHz, and 5,800 K at 1.56

THz for a NbN HEB de vice, cou pled through a sili con lens and a log- periodic an tenna. The very small LO

power re quired by such de vices when op ti mally matched (as low as 100 nW) has been veri fied. We have

also dem on strated 3 dB con ver sion gain of an HEB mixer de vice at 1.56 THz, for a 600 kHz IF fre quency.

Fig ure 8: I-V curves of the de vice ir ra di ated by four dif fer ent fre quen cies.

We ex pect that meas ured re ceiver noise tem pera tures of NbN HEB mix ers will con tinue their down ward

trend at fre quen cies above 1 THz in the fu ture. NbN HEB THz mix ers are es pe cially ad van ta geous in the

fol low ing re spects:

• Sub mi cron size is not re quired

• Can be op er ated at 4.2 K; Al de vices re quire He3 cool ing

• LO power 100 nW, smaller for sub mi cron de vices

• Pre sent re ceiver noise tem pera tures are as low as for diffusion- cooled mix ers up to 1 THz

• Op er at ing point not sen si tive to 300 K ra dia tion

AC KNOW LEDGE MENTSThis work was sup ported by the Rus sian Pro gram on Con densed Mat ter (Su per con duc tiv ity Di vi -

sion) un der Grant No.93169, as well as grants from the Na tional Sci ence Foun da tion (ECS- 96128), The

Na tional Re search Coun cil, and NASA (NRA 93- OSSA- 06). Laser tron, Inc. pro vided ac cess to their IR

mask- aligner. We grate fully ac knowl edge their as sis tance with this cru cial step in our fab ri ca tion pro cess.

We would also like to thank Dr. Neil Er ick son for lend ing us the mul ti plier source used for the 356 GHz

meas ure ments, and Dr. A. Ver evkin for help with ex peri ments at UMASS/Am herst.

VII. REF ER ENCESS. Chered nichenko et al., “Large Band width of NbN Pho non Cooled Hot Elec tron Bo lo me ter Mix ers on Sap phire

Sub strates,” 8th In tern.Symp.Space THz Tech nol., Cam bridge, MA, March 1997.

H. Ek strom, E. Koll berg, P. Ya goubov, G. Gol’ts man, E. Gershen zon, and K.S. Yngves son, “Gain and Noise

Band width of NbN Hot Elec tron Bo lo met ric Mix ers,” Appl. Phys. Lett., 70, 3296, 1997.

H. Ek strom, B. Kara sik, E. Koll berg and S. Yngves son, “Con ver sion Gain and Noise of Nio bium Hot- Electron

Mix ers,” IEEE Trans.Mi crow.The ory Tech niques, MTT- 43, 938 (1995)

D.F. Filipo vic et al., “Double- Slot An ten nas on Ex tended Hemi spheri cal and El lip ti cal Di elec tric Lenses,” IEEE

Trans.Mi cro wave The ory Tech niques, MTT- 41, 1738, 1993.

J. Ka wa mura, R. Blun dell, C.-Y.E. Tong, G. Gol’ts man, E. Gershen zon, B. Vo ro nov, and S. Chered nichenko,

“Phonon- Cooled NbN HEB Mix ers for Sub mil li me ter Wave lengths,” 8th In tern.Symp.Space THz Tech nol.,

Cam bridge, MA, March 1997.

B.K. Kor man yos et al., “A Pla nar Wide band 80- 200 GHz Sub har monic Re ceiver,” IEEE Trans. Mi crow.The ory

Tech niques, MTT- 41, 1730 (1993).

M. Kroug, P. Ya goubov, G. Gol’ts man and E. Koll berg, “NbN Qua siop ti cal Pho non Cooled Hot Elec tron Bo lo met -

ric Mixer at THz Fre quen cies,” EUCAS’97, Eind ho ven, The Neth er lands, June 29- July 3, 1997.

H. Mer kel and E. Koll berg, “A Large Sig nal Model for Phonon- Cooled Hot- Electron Bo lo met ric Mix ers for THz

Ap pli ca tions”, this sym po sium.